The environment in which we live can be considered as having three fundamental sets of components: 

Physical –(Energy of one form or another.

Chemical –(substances whether natural or man-made)

Biological –(Living Things)

Physical:

• Increasing extremes of temperature, as a result of climatic change, could result in increased mortality even in temperate climates. 
• Electromagnetic radiation and ionising radiation. For example- radio,microwaves, infra-red waves, ultra-violet rays,x-rays and gamma rays.
• Exposure to  (UV) radiation carries a increased risk of skin cancer such as melanoma, and of cataracts which are to an extent exposure related.
• Some pollutants such as chlorofluorocarbons (CFCs) used as refrigerants or in aerosol propellants or in the manufacture of certain plastics can damage the “ozone layer” in the higher atmosphere (stratosphere) and thus allow more UV light to reach us, and harm us directly.
• Ultraviolet light may also cause harm indirectly by contributing to an increase in ozone in the troposphere. 
• Radioactivity is associated with an exposure dependent risk of some cancers notably leukaemia.
• Radon gas arising from certain rock types beneath dwellings can contribute to cancer risk.
• Ionising radiation from the nuclear industry and from fallout from detonations contributes to disease like leukaemia.
• The effect of viral infections associated with population shifts. 
• Noise pollution – noisy discotheques and personal stereos can cause deafness.

Chemical Components:

• Tobacco smoke as an environmental hazard  represents the single biggest known airborne chemical risk to health, whether measured in terms of death rates or ill-health (from lung cancer, other lung disease such as chronic bronchitis and emphysema, and disease of the heart, especially, and of blood vessels and other parts of the body). It poses a great risk for non-smokers exposed passively to side stream tobacco smoke. 
• Combustion of coal and other solid fuels can produce smoke (containing polycyclic aromatic hydrocarbons – PAH) and sulphur dioxide besides other agents such as those  produced by: 
• Combustion of liquid petroleum products which can generate carbon monoxide, oxides of nitrogen and other agents.
• Industry and incineration can generate a wide range of products of combustion such as oxides of sulphur and nitrogen, polycyclic aromatic hydrocarbons, dioxins etc.
• Combustion of any fossil fuel generates varying amounts of particulate matter. It also adds to the environmental burden of carbon dioxide – an important “green house” gas .
•  Combustion of fuel can also generate hazardous substances in other ways, besides by chemical oxidation, such as by liberating benzene (from the “cracking” of petrol) or lead (from leaded petrol).
• Some of the primary pollutants such as nitrogen dioxide can, under the influence of UV light generate secondary pollutants notably ozone (an allotrope of oxygen)
• Health effects of concern are asthma, bronchitis and similar lung diseases, and there is good evidence relating an increased risk of symptoms of these diseases with increasing concentration of sulphur dioxide, ozone and other pollutants.
•  Moreover, there is increasing evidence to suggest that pollution from particulate increases the risk of morbidity and mortality from cardiopulmonary disease especially in people with other risk factors (such as old age, or heart and lung disease).  
• Products of combustion and other harmful airborne pollutants can also arise within the home. Thus nitrogen dioxide generated by gas fires or gas cookers can contribute to an increased respiratory morbidity of those living in the houses.
•  Certain modern building materials may liberate gases or vapours such as formaldehyde at low concentration which might provoke mild respiratory and other symptoms in some occupants.
• Large scale industrial releases with serious acute effects are fortunately rare but you might recollect some events such as in Bhopal (India).
•  Various smaller scale events occur such as leaks from road tankers, or fires in warehouses and factories.
• Special local environmental exposures can arise for example in communities exposed to drifting pesticide sprays containing organophosphates.
• Some natural phenomena such as volcanic eruptions can present serious risks to health. Fortunately they are rare but can be catastrophic. 
• Water can be an important source of chemical hazards.
• It can leach lead from pipes especially if the water is soft. There is good epidemiological evidence that this can have harmful effect especially on neurological function. Other adverse effects can arise from chemicals added to the water. 
• Chlorination of water has probably saved millions of lives.
• Some concern has been raised about possible increased cancer risks in association with chlorinated water. Fluoride added to water reduces the risks of caries but can also have unwanted effects such as mottling of the teeth. 
• Nitrate in water usually arising from fertiliser leaching (natural or artificial) can increase the risk of methaemoglobinaemia (‘blue babies’) in bottle fed infants.  Pesticides can and do leach into water.
• Deposition of solid hazardous waste can result in harmful substances leaching into water supplies, becoming airborne or being swallowed or otherwise absorbed directly (for example because of children playing on the sites).
• Iodine 131- It is a product of nuclear fission.I-131 in fallout from nuclear weapons or reactor accidents can occur in particle form, which can be ingested in food or water.It can pass on to vegetation which is consumed by cattle and finally reach human beings through milk.
• Mercury- Mercury is a naturally occurring element that is found in air, water and soil.
• Exposure to mercury – even small amounts – may cause serious health problems, and is a threat to the development of the child in utero and early in life.
• Mercury may have toxic effects on the nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes.
• Mercury is considered by WHO as one of the top ten chemicals or groups of chemicals of major public health concern.
• People are mainly exposed to methyl mercury, an organic compound, when they eat fish and shellfish that contain the compound. High levels of Mercury have been found in fish stocks in coastal areas-Mumbai Kolkatta, Karwar and North Koel in Bihar.

Biological  Component These generally fall into two broad categories: those which produce adverse health effects through infection and those which produce adverse effects in non-infective (allergic) ways. Contamination of drinking water by soiled water (usually coliforms) can cause cholera,outbreak of water borne diseases.Recreational water which is heavily contaminated with pathogens, notably coliform bacteria has been shown to be associated with gastrointestinal and other infectious illness.So-called “clinical” waste is not merely an occupational hazard of health care workers but is becoming an increasingly more important risk, for example for children finding blood stained needles. Many allergens such as grass pollen grains, or faecal material from house dust mites may cause attacks of asthma or “hay fever” (allergic rhinitis).  An increasing number of studies suggest that airborne chemical pollution can act synergistically with naturally occurring allergens and result in effects on lung function . Pesticides Pesticides are the only toxic substances released intentionally into our environment to kill living things. This includes substances that kill weeds (herbicides), insects (insecticides), fungus (fungicides), rodents (rodenticides), and others.The use of toxic pesticides to manage pest problems has become a common practice around the world. Pesticides are used almost everywhere — not only in agricultural fields, but also in homes, parks, schools, buildings, forests, and roads. In addition, pesticides can be found in the air we breathe, the food we eat, and the water we drink.Pesticides are commonly used in India for obvious reasons. 15 different pesticides in the 20 blood samples were tested from four villages in Punjab.Pesticides once ingested tend to accumulate in the body fat and persisit in the body for many years.