What is Mesothelioma?  Mesothelioma is a type of cancer that develops from the thin layer of tissue that covers many of the internal organs, known as the mesothelium. The most common area affected is the lining of the lungs and chest wall.

It is Caused by asbestos, mesothelioma has no known cure and has a very poor prognosis.  Other risk factors include exposure to Radiation, Carbon nanotubes, genetic factors. Prevention centres around reducing exposure to asbestos. Treatment often includes surgery, radiation therapy, and chemotherapy.

KEYNOTE: Prognosis is how a disease will progress. A prognosis can be good or bad, and may include a life expectancy estimate.


Signs and symptoms of mesothelioma may include shortness of breath due to fluid around the lung, a swollen abdomen, chest wall pain, cough, feeling tired, and weight loss.These symptoms typically come on slowly.


Mesothelioma is most commonly classified by the location in the body where it develops. Specifically, cancer forms in the lining of certain organs or spaces within the body, known as the mesothelium. Mesothelioma typically develops in one of three specific areas.

  • Pleural Mesothelioma (LUNGS): The most common type, pleural mesothelioma is caused by the inhalation of asbestos fibres.
  • Peritoneal Mesothelioma (ABDOMEN):  Inhaled or swallowed asbestos fibres can         become trapped in the lining of the abdomen (the peritoneum).
  • Pericardial Mesothelioma (Heart): In rare cases, asbestos fibres can get lodged in the   pericardium, the lining around the heart cavity.

NOTE: Mesothelioma can also be characterised by the type of cell that makes up the tumours. The cell type is determined through a process known as histology, which is a microscopic inspection of the tissue acquired through a biopsy.



Cloning is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually.

Clones are organisms that are exact genetic copies. Every single bit of their DNA(Deoxyribonucleic Acid) is identical.  Cloning is commonly used to amplify DNA fragments containing whole genes, but it can also be used to amplify any DNA sequence such as promoters, non-coding sequences and randomly fragmented DNA. It is used in a wide array of biological experiments and practical applications ranging from genetic fingerprinting to large scale protein production.


                                    CLONING A GENE (Artificial)

When scientists clone an organism, they are making an exact genetic copy of the whole organism.

When scientists clone a gene, they isolate and make exact copies of just one of an organism’s genes. Cloning a gene usually involves copying the DNA sequence of that gene into a smaller, more easily manipulated piece of DNA, such as a plasmid.


clonesSome researchers are looking at cloning as a way to create stem cells that are genetically identical to an individual. These cells could then be used for medical purposes, possibly even for growing whole organs. And stem cells cloned from someone with a disease could be grown in culture and studied to help researchers understand the disease and develop treatments.

Cloning can also occur Naturally;

some plants and single-celled organisms, such as bacteria, produce genetically identical offspring through a process called asexual reproduction. In asexual reproduction, a new individual is generated from a copy of a single cell from the parent organism.

Natural clones, also known as identical twins, occur in humans and other mammals. These twins are produced when a fertilized egg splits, creating two or more embryos that carry almost identical DNA. Identical twins have nearly the same genetic makeup as each other, but they are genetically different from either parent.

KEY NOTEIn 2013, scientists at Oregon Health and Science University were the first to use cloning techniques to successfully create human embryonic stem cells. The donor DNA came from an 8-month-old with a rare genetic disease.






 Epidemiology is the study of the distribution and determinants of health-related states or events (including disease), and the application of this study to the control of diseases and other health problems. Various methods can be used to carry out epidemiological investigations: surveillance and descriptive studies can be used to study distribution; analytical studies are used to study determinants.

It is the cornerstone of public health, and shapes policy decisions and evidence-based practice by identifying risk factors for disease and targets for preventive healthcare.


Basic principles of epidemiology in emergencies

Epidemiology is the study of the causes and distribution of disease in human populations. An epidemiological approach helps planners to focus on the main problems of a community rather than of individual patients and to identify measures for improving the health of the community as a whole.
Epidemiology can increase the general understanding about a disease and particularly how it is transmitted even when the cause is unknown.

 In epidemiology, the assumption is that diseases do not occur randomly, but follow predictable patterns that can be studied and expressed in terms of what, who, where, when, how, why, and what next.



In emergencies, epidemiology has three elements:

1. Descriptive Epidemiology determines the distribution of a disease among displaced populations. It describes the health problem, its frequency, those affected, where, and when. The events of interest are defined in terms of the time period, the place and the population at risk.

Examples: Monitoring the health status of a population to detect cholera cases, such as, by age, sex, location, water source and duration of stay in a dispersed population or camps.
Conducting a nutritional survey to determine the prevalence of acute malnutrition among children under five.

2. Analytical epidemiology compares those who are ill with those who are not in order to identify the risk of disease or protective factors (determinant of a disease). It examines how the event (illness, death, malnutrition, injury) is caused (e.g. environmental and behavioural factors) and why it is continuing. Standard mathematical and statistical procedures are used.

Example: Investigating an outbreak of an unknown disease in a displaced
population settlement.

3. Evaluation epidemiology examines the relevance, effectiveness and impact of different programme activities in relation to the health of the affected populations.

Example: Evaluating a malaria control programme for displaced populations.


Key aspects of epidemiology

A number of other fields – medicine, nursing, dentistry, pharmacy, demography, sociology, health psychology, health education, health policy, nutrition – share many common features and areas of
interest with epidemiology (and with each other).

Some of the key aspects of epidemiology are:
Epidemiology deals with populations, thus involving:
1. Rates and proportions
2. Averages
3. Heterogeneity within
4. Dynamics – demography, environment, lifestyle



Epidemiology has many uses in emergency situations, including:
ƒ 1. Rapid needs assessment;
ƒ2. Demographic studies determining the population size and structure of affected communities in camp settings or dispersed within a host population;
ƒ 3. Population surveys for determining health status (death rates, incidence/prevalence of disease, nutrition and immunisation status) and assessing programme coverage;
ƒ 4. Investigating a disease outbreak;
ƒ 5. Public health surveillance and management information system;
6.ƒ Programme monitoring and evaluation.



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Genetics is the study of genes, heredity, and genetic variation in living organisms. It is generally considered a field of biology, but it intersects frequently with many of the life sciences. The word genetics stems from the Ancient Greek γενετικός genetikos meaning “genitive”/”generative”, which in turn derives from γένεσις genesis meaning “origin”.

Genetic processes work in combination with an organism’s environment and experiences to influence development and behavior, often referred to as nature versus nurture. The intra- or extra-cellular environment of a cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate. While the average height of the two corn stalks may be genetically determined to be equal, the one in the arid climate only grows to half the height of the one in the temperate climate, due to lack of water and nutrients in its environment.

KEY POINTS: The father of genetics is Gregor Mendel, a late 19th-century scientist and Augustinian friar.

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of the two is responsible for inheritance. In 1928, Frederick Griffith discovered the phenomenon of transformation (see Griffith’s experiment): dead bacteria could transfer genetic material to “transform” other still-living bacteria. Sixteen years later, in 1944, the Avery–MacLeod–McCarty experiment identified DNA as the molecule responsible for transformation. The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hämmerling in 1943 in his work on the single celled alga Acetabularia. The Hershey–Chase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.


A genetic disorder is a disease that is caused by an abnormality in an individual’s DNA. Abnormalities can be as small as a single-base mutation in just one gene, or they can involve the addition or subtraction of entire chromosomes.

KEY: DNA is deoxyribonucleic acid. It is the molecule that contains the genetic code of organisms. This includes animals, plants, protists, archaea and bacteria.

Genetic disorders may or may not be heritable, i.e., passed down from the parents’ genes. In non-heritable genetic disorders, defects may be caused by new mutations or changes to the DNA. In such cases, the defect will only be heritable if it occurs in the germ line.


There are different types of genetic disorder, some of which includes:

  1. Single-Gene Disorders: These disorders involve mutations in the DNA sequences of single genes. As a result, the protein the gene codes for is either altered or missing. some examples of single – Gene disorder are;Adenosine deaminase (ADA) deficiency, Alpha-1 Antitrypsin Deficiency,Cystic Fibrosis,Galactosemia,
    Huntington’s Disease, Maple Syrup Urine Disease (MSUD),Neurofibromatosis Type 1,Pachyonychia Congenita , Phenylketonuria, Severe Combined Immunodeficiency , Sickle Cell Disease, Smith-Lemli-Opitz Syndrome .





2. Chromosomal Abnormalities: In these disorders, entire chromosomes, or large segments of them, are missing, duplicated, or otherwise altered. Examples include; Cri-du-Chat Syndrome, Down Syndrome, 47, XXY (Klinefelter syndrome),Turner Syndrome, and Williams Syndrome.








3. Multifactorial Disorders: These disorders involve variations in multiple genes, often coupled with environmental causes. Examples include; Diabetes, Hypertension, Alzheimers Disease, Breast/Ovarian Cancer, Colon Cancer, Hypothyroidism, etc.



INTRODUCTION: Your digestive tract stretches from your mouth to your anus. It includes the organs necessary to digest food and process waste.

The human gastrointestinal tract, or GI tract, or GIT is an organ system responsible for consuming and digesting foodstuffs, absorbing nutrients, and expelling waste.
The tract consists of the stomach and intestines, and is divided into the upper and lower gastrointestinal tracts.

Do you know that The whole digestive tract is about nine metres (30 feet) long?

The GI tract releases hormones from
enzymes to help regulate the digestive process. These hormones, including
gastrin, secretin, cholecystokinin , and
ghrelin, are mediated through either
intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution .


Nutrients from the GI tract are not processed on-site; they are taken to the liver to be broken down further, stored, or distributed.

The Penalty for Ignorance is Lack


cell theory is a scientific theory which describes the properties of
cells. These cells are the basic unit of structure in all organisms and also the basic unit of reproduction.

This discovery is largely attributed to Robert Hooke , and began the scientific study of cells, also known as cell biology.
The discovery of the cell was made possible through the invention of the microscope.

Tenet is one of the principles on which a belief or theory is based.
The three tenets to the cell theory are as described below:
1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and organization in organisms.
3. Cells come from preexisting cells.
The modern version of the Cell. Theory includes the ideas that:
•Energy flow occurs within cells.
•Heredity information (DNA) is passed on from cell to cell.
•All cells have the same basic chemical composition.

Do You Know that Cells Commit Suicide?
When a cell becomes damaged or undergoes some type of infection, it will self destruct by a process called
apoptosis . Apoptosis works to ensure proper development and to keep the body’s natural process of mitosis in check. A cell’s inability to undergo apoptosis can result in the development of cancer .

Eukaryotic and prokaryotic cells are the two main types of cells. Eukaryotic cells are called so because they have a true
nucleus . Animals, plants, fungi and protists are examples of organisms that are composed of eukaryotic cells.
Prokaryotes include bacteria and

The Penalty for Ignorance is Lack



The cell (from Latin cella, meaning “small room” is the basic structural, functional, and biological unit of all known living organisms. Cells are the smallest unit of life that can replicate independently, and are often called the “building blocks of life”. The study of cells is called cell biology.
Cells consist of cytoplasm enclosed within a membrane , which contains many biomolecules such as proteins and
nucleic acids.  Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or
multicellular (including plants and
animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (10 13 ) cells.  Most plant and animal cells are visible only under the microscope, with dimensions between 1 and 100 micrometres .

The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by
Christian monks in a monastery.



To survive in a dynamic world, cells evolved mechanisms for adjusting their organic chemistry in response to signals indicating environmental modification.
The changes will take several forms, as well as changes within the activities of pre-existing enzymne molecules, changes within the rates of synthesis of latest protein molecules, and changes in membrane-transport processes.
Chemicals that might pass into cells, either by diffusion through the cell membrane or by the action of transport protein and will bind on to protein within the cell and modulate their activities

Passive Transport: Movement across the cell membrane that doesn’t need energy from the cell.
Concentration Gradient: A distinction within the concentration of a substance across an area.
Equilibrium: A condition within which the concentration of a substance is equal throughout an area.
Diffusion: The movement of a substance from a region of high concentration to a region of lower concentration caused by the random motions of particles of the substance.
Osmosis: The diffusion of water through a by selective semi permeable membrane.
Hypertonic Solution: A solution that causes a cell to shrink due to osmosis.
Hypotonic Solution: A solution that causes a cell to swell due to osmosis.
Isotonic Solution: A solution that produces no modification in cell volume due to osmosis.
Ion Channel: A transport protein w/a polar pore that ions can pass through
Carrier Proteins: A transport protein which will bind to a selected substance on one side of a membrane, and free it on the opposite side.


The Cell in its Environment-Physical and Biophysical Processes
Substances can move into and out of a cell through its semi-permeable cell membrane. There are three different processes through which materials can move in and out of a cell. They are:
• Through the process of diffusion,
• Through the process of osmosis and
• Through the process of active transport

The Penalty for Ignorance is Lack


1. Heredity:
Heredity is a biological process through which the transmission of physical and social characteristics takes place from parents to off-springs. It greatly influences the different aspects of growth and development i.e. height, weight and structure of the body, colour of hair and eye, intelligence, aptitudes and instincts.
However environment equally influences the above aspects in many cases. Biologically speaking heredity is the sum total of traits potentially present in the fertilized ovum (Combination of sperm cell & egg cell), by which off-springs are resemblance to their parents and fore parents.
2. Environment
Environment plays an important role in human life. Psychologically a person’s environment consists of the sum total of the stimulations (physical & Psychological) which he receives from his conception. There are different types of environment such as physical, environment, social environment & psychological environment.
Physical environment consists of all outer physical surroundings both in-animate and animate which have to be manipulated in order to provide food, clothing and shelter. Geographical conditions i.e. weather and climates are physical environment which has considerable impact on individual child.
Social environment is constituted by the society-individuals and institutions, social laws, customs by which human behavior is regulated.
Psychological environment is rooted in individual’s reaction with an object. One’s love, affection and fellow feeling attitude will strengthen human bond with one another.
So Growth and Development are regulated by the environment of an individual where he lives.
3. Sex
Sex acts as an important factor of growth and development. There is difference in growth and development of boys and girls. The boys in general taller, courageous than the girls but Girls show rapid physical growth in adolescence and excel boys. In general the body constitution and structural growth of girls are different from boys. The functions of boys and girls are also different in nature.
4. Nutrition
Growth and Development of the child mainly depend on his food habits & nutrition. The malnutrition has adverse effect on the structural and functional development of the child.
5. Races
The racial factor has a great influence on height, weight, colour, features and body constitution. A child of white race will be white & tall even hair and eye colour, facial structure are governed by the same race.
6. Exercise
This does not mean the physical exercise as a discipline. The functional activities of the child come in the fold of exercise of the body. We do not mean any law of growth through use or atrophy (The reverse of growth) through disuse.
The growth of muscles from the normal functioning of the child is a matter of common knowledge. It is a fact that repeated play and rest build the strength of the muscle. The increase in muscular strength is mainly dye to better circulation and oxygen supply. The brain muscles develop by its own activity-play and other activities provide for these growth and development of various muscles. Deliberately the child does not play or engages himself in various other functions with the knowledge that they will help him in growing. This style of functioning of the child is but natural.
7. Hormones
There are a number of endocrine glands inside the human body. Endocrine glands are ductless glands. This means there are certain glands situated in some specific parts of the body. These glands make internal secretions locally. These secretions produce one or more hormones.
Hormones are physiological substances having the power to raise or lower the activity level of the body or certain organs of the body. For example, the gland pancreas secretes pancreatic juice, not into the blood, but into the intestine. Here it acts upon food and plays an important part in digestion of food. This pancreas also discharges into the blood, a substance called insulin. This being carried by the blood to the muscles enables them to use sugar as a fuel to add strength to muscles. It the pancreas fails to produce the secretions, the organism lapses to the unfavorable conditions of growth and development.
Similarly, the adrenal glands are very close to kidneys. These make a secretion of adrenaline, a very powerful hormone, which is responsible for strong and rapid heart-beat, release of stored sugar from liver and which controls blood pressure. Gonads are glands, which secrete hormones that have important effects on growth and sex behavior.
A balance of male hormones controls development in the direction of masculinity and that of female hormones steers it toward feminist. At puberty, these sex hormones promote the development of genital organs. Lacking the gonads, individuals of either sex develops into rather a neutral specimen without strong sex characteristics. Pituitary is called the “master gland”. It is attached to the under side of the brain and its secretions controls the brain function and also the blood pressure. It stimulates other glands like adrenal and gonads. If this gland is over-active in childhood, the muscles and bones grow very rapidly and the individual may become a giant of seven to nine feet tall.
8. Learning and Reinforcement
Learning is the most important and fundamental topic in the whole science of psychology. Development consists of maturation and learning. Without any learning the human organism is a structure of various limbs, all other internal organs with muscles and bones. But it is not human being with maturation.
Learning includes much more than school learning. Learning goes to help the human child in his physical, mental, emotional, intellectual, social and attitudinal developments. All knowledge and skill, all habits, good and bad, all acquaintances with people and things, all attitudes built up in your dealing with people and things have been learned.
Reinforcement is a factor in learning. Exercise or activity is necessary for learning. It may be a motor activity, as in playing on a musical instrument. Or it may be a sensory activity as in listening to a piece of music. Whatsoever, there most be activity in some from. “We learn by doing”. It is an old psychological proverb. Now it is that out activity should be repeated till we get the desired results. So the proverb should be, “We learn by doing getting results.”

The Penalty for Ignorance is Lack



Cellular respiration is the set of metabolic
reactions and processes that take place in the
cells of organisms to convert biochemical
energy from nutrients into adenosine
triphosphate (ATP), and then release waste

Once the energy that was in sunlight is
transformed into chemical energy, often by
photosynthesis , the organism has to now convert
the chemical energy into a usable form. It may
seem a bit odd for there still to be more steps.
After all, when you eat a candy bar isn’t the sugar
in the candy bar “burnt” by the body to provide
energy? Well the answer is yes and no. First of all
when we burn something normally in the air we
combine that substance with oxygen releasing
energy from the substance. Indeed, an analogous
process does happen in our bodies.
What goes on in living things is not really like
burning because the molecules from which we
harvest energy give up their energy in a controlled
fashion rather than all at once as what happens in
a fire. Think of your car. All the energy in the gas
tank when you get in your car is not released all at
once but rather in small bursts which allow you to
control the car’s movement. In the same way cells
take the energy from the “food” and package that
energy into manageable bursts that provide just
the right amount of energy for the organism’s
activities be those activities driving a car or
flashing a light to attract a mate.


The point of cellular respiration is to harvest
electrons from organic compounds such as
glucose and use that energy to make a molecule
called ATP. ATP in turn is used to provide energy
for most of the immediate work that the cell does.
ATP can be thought of as being like a small
package of energy that has just the right amount of
energy that can be used in a controlled manner.
ATP: Adenosine tri-phosphate,ATP is a nucleotide
with three phosphate groups instead of one
phosphate group. The point of cellular respiration
is to harvest chemical energy from food and store
it in the chemical bonds of ATP.


Types of cellular respiration: There are two basic types of cellular respiration aerobic cellular respiration and anaerobic cellular respiration. Aerobic respiration requires the use of oxygen and anaerobic respiration which does not use oxygen. There are several types of anaerobic respiration, most familiar is a process called fermentation.

Aerobic Respiration.

Aerobic respiration is the process by which ATP is produced by cells by the complete oxidation of organic compounds using oxygen . In aerobic respiration oxygen serves as the final electron acceptor, accepting electrons that ultimately come from the energy rich organic compounds we consume. We will use glucose as an illustration of an organic molecule used in cellular respiration since glucose is a common energy source for cells. In this figure the energy rich molecules (and heat) are in red, energy poor molecules(relatively speaking) are in black.

VBS Home page,VBS Course Navigator, Cellular Respiration, Overview, Previous Page, Next Page , Top of page

Stages in Aerobic Respiration:

Aerobic Respiration takes place in three stages which are summarized here starting with the original glucose molecule.

Glycolysis. Glycolysis is the first step in cellular respiration and all cells regardless of the type of cellular respiration they do are able to carry out glycolysis. Because of this we believe that glycolysis probably arose very early in the evolution of life on the planet. In glycolysis glucose is partially oxidized and broken down into two 3 carbon molecules called pyruvate or pyruvic acid. In the process, glycolysis produced 4 ATP for a net gain of two ATP and two molecules of NADH. Each NADH is carrying two energy rich electrons away from the glucose and these electrons can be used by the cell to do work.

After glycolysis the pyruvate is processed to harvest 2 more NADH molecules and remove one carbon per pyruvate. The carbon and two oxygens is removed since it no longer has any useful energy. So it is waste. This little step is the source of some of the carbon dioxide we produce.


Role of oxygen in Aerobic Respiration. As the energy rich electrons from food are used to make ATP by electron transport phosphorylation they loose energy and once they are no longer useful they have to be removed. Oxygen is a great electron acceptor and so the electrons are combined with hydrogen ions and oxygen to make water. This prevents electrons from building up in the electron transport system.


Anaerobic respiration is a form of respiration using
electron acceptors other than oxygen. Although
oxygen is not used as the final electron acceptor,
the process still uses a respiratory electron
transport chain ; it is respiration without oxygen. In
order for the electron transport chain to function,
an exogenous final electron acceptor must be
present to allow electrons to pass through the
system. In aerobic organisms, this final electron
acceptor is oxygen. Molecular oxygen is a highly
oxidizing agent and, therefore, is an excellent
acceptor. In anaerobes, other less-oxidizing
substances such as sulfate (SO 4 2−), nitrate (NO 3−)
, sulphur (S), or fumarate are used. These terminal
electron acceptors have smaller reduction
potentials than O 2, meaning that less energy is
released per oxidized molecule. Anaerobic
respiration is, therefore, in general energetically
less efficient than aerobic respiration.

Anaerobic respiration is used mainly by
prokaryotes that live in environments devoid of
oxygen. Many anaerobic organisms are obligate
anaerobes meaning that they can respire only
using anaerobic compounds and will die in the
presence of oxygen.


Cellular respiration (both aerobic and anaerobic)
utilizes highly reduced species such as NADH and
FADH2 (for example produced during glycolysis
and the citric acid cycle ) to establish an
electrochemical gradient (often a proton gradient)
across a membrane, resulting in an electrical
potential or ion concentration difference across the
membrane. The reduced species are oxidized by a
series of respiratory integral membrane proteins
with sequentially increasing reduction potentials
with the final electron acceptor being oxygen (in
aerobic respiration ) or another species (in
anaerobic respiration). The membrane in question
is the inner mitochondrial membrane in eukaryotes
and the cell membrane in prokaryotes . A proton
motive force or pmf drives protons down the
gradient (across the membrane) through the
proton channel of ATP synthase . The resulting
current drives ATP synthesis from ADP and
inorganic phosphate.
Fermentation , in contrast, does not utilize an
electrochemical gradient. Fermentation instead
only uses substrate-level phosphorylation to
produce ATP. The electron acceptor NAD+ is
regenerated from NADH formed in oxidative steps
of the fermentation pathway by the reduction of
oxidized compounds. These oxidized compounds
are often formed during the fermentation pathway
itself, but may also be external. For example, in
homofermentative lactic acid bacteria, NADH
formed during the oxidation of glyceraldehyde-3-
phosphate is oxidized back to NAD+ by the
reduction of pyruvate to lactic acid at a later stage
in the pathway. In yeast , acetaldehyde is reduced
to ethanol to regenerate NAD+.


Atmosphere of Earth

The atmosphere of Earth is a layer of gases
surrounding the planet Earth that is retained by
Earth’s gravity. The atmosphere protects life on
Earth by absorbing ultraviolet solar radiation ,
warming the surface through heat retention
(greenhouse effect ), and reducing temperature
extremes between day and night (the diurnal
temperature variation).
The common name given to the atmospheric gases
used in breathing and photosynthesis is air . By
volume, dry air contains 78.09% nitrogen, 20.95%
oxygen , 0.93% argon , 0.039% carbon dioxide ,
and small amounts of other gases. Air also
contains a variable amount of water vapor, on
average around 1% at sea level, and 0.4% over the
entire atmosphere. Although air content and
atmospheric pressure vary at different layers, air
suitable for the survival of terrestrial plants and
terrestrial animals currently is only known to be
found in Earth’s troposphere and artificial
atmospheres .
The atmosphere has a mass of about 5.15×10 18
kg, [2] three quarters of which is within about 11
km (6.8 mi; 36,000 ft) of the surface. The
atmosphere becomes thinner and thinner with
increasing altitude, with no definite boundary
between the atmosphere and outer space. The
Kármán line , at 100 km (62 mi), or 1.57% of
Earth’s radius, is often used as the border between
the atmosphere and outer space. Atmospheric
effects become noticeable during atmospheric
reentry of spacecraft at an altitude of around 120
km (75 mi). Several layers can be distinguished in
the atmosphere, based on characteristics such as
temperature and composition.
The study of Earth’s atmosphere and its processes
is called atmospheric science or aerology . Early
pioneers in the field include Léon Teisserenc de
Bort and Richard Assmann .
Main article: Atmospheric chemistry
Mean atmospheric water vapor
Air is mainly composed of nitrogen, oxygen , and
argon , which together constitute the major gases of
the atmosphere. Water vapor accounts for roughly
0.25% of the atmosphere by mass. The
concentration of water vapor (a greenhouse gas)
varies significantly from around 10 ppmv in the
coldest portions of the atmosphere to as much as
5% by volume in hot, humid air masses, and
concentrations of other atmospheric gases are
typically provided for dry air without any water
vapor. The remaining gases are often referred to
as trace gases, among which are the
greenhouse gases such as carbon dioxide,
methane, nitrous oxide, and ozone. Filtered air
includes trace amounts of many other chemical
compounds . Many substances of natural origin
may be present in locally and seasonally variable
small amounts as aerosols in an unfiltered air
sample, including dust of mineral and organic
composition, pollen and spores, sea spray, and
volcanic ash . Various industrial pollutants also
may be present as gases or aerosols, such as
chlorine (elemental or in compounds), fluorine
compounds and elemental mercury vapor. Sulfur
compounds such as hydrogen sulfide and sulfur
dioxide (SO 2 ) may be derived from natural sources
or from industrial air pollution .
Major constituents of dry air, by volume
in ppmv(B)
in %
N 2
O 2
Carbon dioxide
CO 2
CH 4
Not included in above dry atmosphere:
Water vapor (C)
H 2O
10–50,000 (D)
0.001%–5% (D)
(A) volume fraction is equal to mole fraction for
ideal gas only,
also see volume (thermodynamics)
(B) ppmv: parts per million by volume
(C) Water vapor is about 0.25% by mass over full
(D) Water vapor strongly varies locally[4]
Structure of the atmosphere
Principal layers
In general, air pressure and density decrease with
altitude in the atmosphere. However, temperature
has a more complicated profile with altitude, and
may remain relatively constant or even increase
with altitude in some regions (see the temperature
section, below). Because the general pattern of the
temperature/altitude profile is constant and
recognizable through means such as balloon
soundings, the temperature behavior provides a
useful metric to distinguish between atmospheric
layers. In this way, Earth’s atmosphere can be
divided (called atmospheric stratification) into five
main layers. Excluding the exosphere, Earth has
four primary layers, which are the troposphere,
stratosphere, mesosphere, and thermosphere.
From highest to lowest, the five main layers are:
Exosphere: 700 to 10,000 km (440 to 6,200
Thermosphere: 80 to 700 km (50 to 440 miles)

Mesosphere: 50 to 80 km (31 to 50 miles)
Stratosphere: 12 to 50 km (7 to 31 miles)
Troposphere: 0 to 12 km (0 to 7 miles) [9]
Earth’s atmosphere Lower 4 layers of the
atmosphere in 3 dimensions as seen diagonally
from above the exobase. Layers drawn to scale,
objects within the layers are not to scale. Aurorae
shown here at the bottom of the thermosphere
can actually form at any altitude in this
atmospheric layer
Main article: Exosphere
The exosphere is the outermost layer of Earth’s
atmosphere (i.e. the upper limit of the atmosphere)
. It extends from the exobase, which is located at
the top of the thermosphere at an altitude of about
700 km above sea level, to about 10,000 km
(6,200 mi; 33,000,000 ft). The exosphere merges
with the emptiness of outer space, where there is
no atmosphere.
This layer is mainly composed of extremely low
densities of hydrogen, helium and several heavier
molecules including nitrogen, oxygen and carbon
dioxide closer to the exobase. The atoms and
molecules are so far apart that they can travel
hundreds of kilometers without colliding with one
another. Thus, the exosphere no longer behaves
like a gas, and the particles constantly escape into
space. These free-moving particles follow ballistic
trajectories and may migrate in and out of the
magnetosphere or the solar wind .
The exosphere is located too far above Earth for
any meteorological phenomena to be possible.
However, the aurora borealis and aurora australis
sometimes occur in the lower part of the
exosphere, where they overlap into the
thermosphere. The exosphere contains most of the
satellites orbiting Earth.
Main article: Thermosphere
The thermosphere is the second-highest layer of
Earth’s atmosphere. It extends from the
mesopause (which separates it from the
mesosphere) at an altitude of about 80 km (50 mi;
260,000 ft) up to the thermopause at an altitude
range of 500–1000 km (310–620 mi; 1,600,000–
3,300,000 ft). The height of the thermopause varies
considerably due to changes in solar activity.
Because the thermopause lies at the lower
boundary of the exosphere, it is also referred to as
the exobase. The lower part of the thermosphere,
from 80 to 550 kilometres (50 to 342 mi) above
Earth’s surface, contains the ionosphere.
This atmospheric layer undergoes a gradual
increase in temperature with height. Unlike the
stratosphere, wherein a temperature inversion is
due to the absorption of radiation by ozone, the
inversion in the thermosphere occurs due to the
extremely low density of its molecules. The
temperature of this layer can rise as high as 1500
°C (2700 °F), though the gas molecules are so far
apart that its temperature in the usual sense is not
very meaningful. The air is so rarefied that an
individual molecule (of oxygen , for example)
travels an average of 1 kilometre (0.62 mi; 3300 ft)
between collisions with other molecules. [10] Even
though the thermosphere has a very high
proportion of molecules with immense amounts of
energy, the thermosphere would still feel extremely
cold to a human in direct contact because the total
energy of its relatively few number of molecules is
incapable of transferring an adequate amount of
energy to the skin of a human. In other words, a
person would not feel warm because of the
thermosphere’s extremely low pressure.
This layer is completely cloudless and free of water
vapor. However non-hydrometeorological
phenomena such as the aurora borealis and aurora
australis are occasionally seen in the
thermosphere. The International Space Station
orbits in this layer, between 320 and 380 km (200
and 240 mi).
Main article: Mesosphere
The mesosphere is the third highest layer of
Earth’s atmosphere, occupying the region above
the stratosphere and below the thermosphere. It
extends from the stratopause at an altitude of
about 50 km (31 mi; 160,000 ft) to the mesopause
at 80–85 km (50–53 mi; 260,000–280,000 ft)
above sea level.
Temperatures drop with increasing altitude to the
mesopause that marks the top of this middle layer
of the atmosphere. It is the coldest place on Earth
and has an average temperature around −85 °C
(−120 °F ; 190 K). [11][12]
Just below the mesopause, the air is so cold that
even the very scarce water vapor at this altitude
can be sublimated into polar-mesospheric
noctilucent clouds . These are highest clouds in the
atmosphere and may be visible to the naked eye if
sunlight reflects off them about an hour or two
after sunset or a similar length of time before
sunrise. They are most readily visible when the
Sun is around 4 to 16 degrees below the horizon.
A type of lightning referred to as either sprites or
ELVES, occasionally form far above tropospheric
thunderclouds. The mesosphere is also the layer
where most meteors burn up upon atmospheric
entrance. It is too high above Earth to be
accessible to jet-powered aircraft, and too low to
support satellites and orbital or sub-orbital
spacecraft. The mesosphere is mainly accessed by
rocket-powered aircraft and unmanned sounding
Main article: Stratosphere
The stratosphere is the second-lowest layer of
Earth’s atmosphere. It lies above the troposphere
and is separated from it by the tropopause . This
layer extends from the top of the troposphere at
roughly 12 km (7.5 mi; 39,000 ft) above Earth’s
surface to the stratopause at an altitude of about
50 to 55 km (31 to 34 mi; 164,000 to 180,000 ft).
The atmospheric pressure at the top of the
stratosphere is roughly 1/1000 the pressure at sea
level. It contains the ozone layer, which is the part
of Earth’s atmosphere that contains relatively high
concentrations of that gas. The stratosphere
defines a layer in which temperatures rise with
increasing altitude. This rise in temperature is
caused by the absorption of ultraviolet radiation
(UV) radiation from the Sun by the ozone layer ,
which restricts turbulence and mixing. Although
the temperature may be −60 °C (−76 °F; 210 K) at
the tropopause, the top of the stratosphere is
much warmer, and may be near 0 °C.
The stratospheric temperature profile creates very
stable atmospheric conditions, so the stratosphere
lacks the weather-producing air turbulence that is
so prevalent in the troposphere. Consequently, the
stratosphere is almost completely free of clouds
and other forms of weather. However, polar
stratospheric or nacreous clouds are occasionally
seen in the lower part of this layer of the
atmosphere where the air is coldest. This is the
highest layer that can be accessed by jet-powered
Main article: Troposphere
The troposphere is the lowest layer of Earth’s
atmosphere. It extends from Earth’s surface to an
average height of about 12 km, although this
altitude actually varies from about 9 km (30,000 ft)
at the poles to 17 km (56,000 ft) at the equator ,
with some variation due to weather. The
troposphere is bounded above by the tropopause ,
a boundary marked by stable temperatures.
Although variations do occur, the temperature
usually declines with increasing altitude in the
troposphere because the troposphere is mostly
heated through energy transfer from the surface.
Thus, the lowest part of the troposphere (i.e.
Earth’s surface) is typically the warmest section of
the troposphere. This promotes vertical mixing
(hence the origin of its name in the Greek word
τρόπος, tropos, meaning “turn”). The troposphere
contains roughly 80% of the mass of Earth’s
atmosphere. The troposphere is denser than all
its overlying atmospheric layers because a larger
atmospheric weight sits on top of the troposphere
and causes it to be most severely compressed.
Fifty percent of the total mass of the atmosphere is
located in the lower 5.6 km (18,000 ft) of the
troposphere. It is primarily composed of nitrogen
(78%) and oxygen (21%) with only small
concentrations of other trace gases.
Nearly all atmospheric water vapor or moisture is
found in the troposphere, so it is the layer where
most of Earth’s weather takes place. It has
basically all the weather-associated cloud genus
types generated by active wind circulation,
although very tall cumulonimbus thunder clouds
can penetrate the tropopause from below and rise
into the lower part of the stratosphere. Most
conventional aviation activity takes place in the
troposphere, and it is the only layer that can be
accessed by propeller-driven aircraft.
Space Shuttle Endeavour orbiting in the
thermosphere. Because of the angle of the photo, it
appears to straddle the stratosphere and
mesosphere that actually lie more than 250 km
below. The orange layer is the troposphere , which
gives way to the whitish stratosphere and then the
blue mesosphere.
Other layers
Within the five principal layers that are largely
determined by temperature, several secondary
layers may be distinguished by other properties:
The ozone layer is contained within the
stratosphere. In this layer ozone concentrations
are about 2 to 8 parts per million, which is
much higher than in the lower atmosphere but
still very small compared to the main
components of the atmosphere. It is mainly
located in the lower portion of the stratosphere
from about 15–35 km (9.3–21.7 mi; 49,000–
115,000 ft), though the thickness varies
seasonally and geographically. About 90% of the
ozone in Earth’s atmosphere is contained in the
The ionosphere is a region of the atmosphere
that is ionized by solar radiation. It is
responsible for auroras . During daytime hours,
it stretches from 50 to 1,000 km (31 to 621 mi;
160,000 to 3,280,000 ft) and includes the
mesosphere, thermosphere, and parts of the
exosphere. However, ionization in the
mesosphere largely ceases during the night, so
auroras are normally seen only in the
thermosphere and lower exosphere. The
ionosphere forms the inner edge of the
magnetosphere . It has practical importance
because it influences, for example, radio
propagation on Earth.
The homosphere and heterosphere are defined
by whether the atmospheric gases are well
mixed. The surfaced-based homosphere
includes the troposphere, stratosphere,
mesosphere, and the lowest part of the
thermosphere, where the chemical composition
of the atmosphere does not depend on
molecular weight because the gases are mixed
by turbulence. This relatively homogeneous
layer ends at the turbopause found at about 100
km (62 mi; 330,000 ft), which places it about 20
km (12 mi; 66,000 ft) above the mesopause.
Above this altitude lies the heterosphere, which
includes the exosphere and most of the
thermosphere. Here, the chemical composition
varies with altitude. This is because the distance
that particles can move without colliding with
one another
is large compared with the size of motions that
cause mixing. This allows the gases to stratify
by molecular weight, with the heavier ones, such
as oxygen and nitrogen, present only near the
bottom of the heterosphere. The upper part of
the heterosphere is composed almost completely
of hydrogen, the lightest element.
The planetary boundary layer is the part of the
troposphere that is closest to Earth’s surface
and is directly affected by it, mainly through
turbulent diffusion . During the day the planetary
boundary layer usually is well-mixed, whereas
at night it becomes stably stratified with weak
or intermittent mixing. The depth of the
planetary boundary layer ranges from as little as
about 100 meters on clear, calm nights to 3000
m or more during the afternoon in dry regions.
The average temperature of the atmosphere at
Earth’s surface is 14 °C (57 °F; 287 K) or 15
°C (59 °F; 288 K), depending on the reference.
Physical properties
Comparison of the 1962 US Standard Atmosphere
graph of geometric altitude against air density ,
pressure , the speed of sound and temperature with
approximate altitudes of various objects.
Pressure and thickness
Main article: Atmospheric pressure
The average atmospheric pressure at sea level is
defined by the International Standard Atmosphere
as 101325 pascals (760.00 Torr; 14.6959 psi ;
760.00 mmHg ). This is sometimes referred to as a
unit of standard atmospheres (atm) . Total
atmospheric mass is 5.1480×10 18 kg
(1.135×10 19 lb), about 2.5% less than would
be inferred from the average sea level pressure and
Earth’s area of 51007.2 megahectares, this portion
being displaced by Earth’s mountainous terrain.
Atmospheric pressure is the total weight of the air
above unit area at the point where the pressure is
measured. Thus air pressure varies with location
and weather .
If the entire mass of the atmosphere had a uniform
density from sea level, it would terminate abruptly
at an altitude of 8.50 km (27,900 ft). It actually
decreases exponentially with altitude, dropping by
half every 5.6 km (18,000 ft) or by a factor of 1/e
every 7.64 km (25,100 ft), the average scale height
of the atmosphere below 70 km (43 mi; 230,000 ft)
. However, the atmosphere is more accurately
modeled with a customized equation for each layer
that takes gradients of temperature, molecular
composition, solar radiation and gravity into
In summary, the mass of Earth’s atmosphere is
distributed approximately as follows:
50% is below 5.6 km (18,000 ft).
90% is below 16 km (52,000 ft).
99.99997% is below 100 km (62 mi; 330,000 ft),
the Kármán line . By international convention,
this marks the beginning of space where human
travelers are considered astronauts .
By comparison, the summit of Mt. Everest is at
8,848 m (29,029 ft); commercial airliners typically
cruise between 10 km (33,000 ft) and 13 km
(43,000 ft) where the thinner air improves fuel
economy; weather balloons reach 30.4 km
(100,000 ft) and above; and the highest X-15 flight
in 1963 reached 108.0 km (354,300 ft).
Even above the Kármán line, significant
atmospheric effects such as auroras still occur.
Meteors begin to glow in this region though the
larger ones may not burn up until they penetrate
more deeply. The various layers of Earth’s
ionosphere , important to HF radio propagation,
begin below 100 km and extend beyond 500 km.
By comparison, the International Space Station and
Space Shuttle typically orbit at 350–400 km, within
the F-layer of the ionosphere where they encounter
enough atmospheric drag to require reboosts
every few months. Depending on solar activity,
satellites can experience noticeable atmospheric