An environmental hazard is a substance, state or event which has the potential to threaten the surrounding natural environment and/or adversely affect human's health. This term incorporates topics like pollution, natural disasters and human-made hazards. Health studies investigate the human health effects of exposure to environmental hazards ranging from chemical pollutants to natural, technological or terrorist disasters. The environment in which we live can be considered as having three fundamental sets of components, physical, chemical, biological. Associations between an exposure and an adverse health effect do not, on their own, prove that the former is the cause of the latter. Many other non-causal associations could explain the findings. Physical hazards involve environmental hazards that can cause harm with or without contact. Examples are earthquakes, electromagnetic fields, floods, light pollution, noise pollution, vibration, x-rays etc. Radioactivity is associated with an exposure dependent risk of some cancers notably leukaemia. The scientific evidence of adverse health effects from general environmental exposure to these fields is "not proven". If there are adverse effects yet to be proven, the risk is probably likely to be small. Chemical substances cause significant damage to the environment. Tobacco smoke is the single biggest known airborne chemical risk to health, whether measured in terms of death rates or ill-health. To a much lesser degree of risk, these adverse effects apply to non-smokers exposed passively to sidestream tobacco smoke. Health effects of concern are asthma, bronchitis, lung cancer 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. Biohazards generally fall into two broad categories: those which produce adverse health effects through infection (microorganisms, viruses or toxins) and those which produce adverse effects in non-infective (allergic) ways.
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