# Environmental Factors in Disease Health and environment are closely interrelated because human health is influenced not only by genetic and biological conditions but also by the physical, chemical, biological, social and built environment in which people live. Disease often develops through the interaction of an individual with environmental conditions. Environmental factors can create favourable conditions for the emergence, transmission and persistence of diseases, while unhealthy environments can weaken human resistance and increase vulnerability. Thus, understanding disease requires attention to the relationship between environmental determinants and health. The physical environment is an important determinant of disease. Air, water, soil, climate, temperature, humidity, housing and sanitation directly affect human health. Air pollution is one of the major environmental causes of illness. Pollutants such as particulate matter, sulphur dioxide, nitrogen oxides, carbon monoxide and ground-level ozone can enter the respiratory system and cause irritation and inflammation. Long-term exposure may contribute to asthma, chronic obstructive pulmonary disease, respiratory infections, cardiovascular diseases and lung cancer. Children, older people and individuals with pre-existing illnesses are particularly vulnerable to polluted air. Water quality is another major environmental determinant of disease. Contaminated drinking water may contain bacteria, viruses, parasites, toxic chemicals and heavy metals. Poor water quality is associated with diseases such as cholera, typhoid, diarrhoea, hepatitis and various parasitic infections. Inadequate sanitation allows human waste to contaminate surface and groundwater, creating conditions for the transmission of infectious diseases. Therefore, safe drinking water, proper sewage disposal and adequate sanitation are essential components of disease prevention. The biological environment also plays an important role in determining patterns of disease. Microorganisms such as bacteria, viruses, fungi and parasites are natural components of the environment, but some cause infectious diseases when they enter the human body. Environmental conditions influence their survival, multiplication and transmission. For example, stagnant water provides breeding sites for mosquitoes that transmit malaria, dengue, chikungunya and other vector-borne diseases. Changes in temperature, rainfall and humidity can alter mosquito populations and consequently affect the geographical distribution of these diseases. Climate is closely connected with human health. Temperature extremes can directly produce illness. Excessive heat may result in dehydration, heat exhaustion and heat stroke, while extreme cold can increase the risk of hypothermia and cardiovascular and respiratory problems. Climate change can further influence disease patterns by changing rainfall, temperature and ecological conditions. Warmer temperatures and altered precipitation may expand the habitat of disease vectors and increase the risk of some infectious diseases. Climate-related disasters such as floods, droughts and cyclones can also cause injuries, displacement, food insecurity, water contamination and outbreaks of infectious diseases. The chemical environment can contribute significantly to both acute and chronic diseases. Human beings may be exposed to pesticides, industrial chemicals, heavy metals, solvents and other hazardous substances through air, water, food and occupational activities. Exposure to lead can adversely affect the nervous system, particularly in children. Mercury and