Throughout the environment—both natural and human built—there are factors that affect human health. Some are naturally occurring hazards, while others are caused by human activities. For example, allergens and irritants in the air (such as pollen and cigarette smoke) can trigger asthma, and exposure to excess radiation or toxic chemicals can cause cancer. Physical, chemical, and biological hazards may be present in all types of media, including air, water, soil, and food. The branch of public health concerned with how these environmental factors affect human health is referred to as environmental health.

The average life expectancy today is significantly longer than it was a century ago. This change in life span is in large part due to improvements in environmental health (such as cleaner drinking water and better sanitation). But along with the advances that have improved environmental health are developments in technology that pose new health hazards. Industrialization—including changes in transportation systems, the manufacturing of consumer goods, and power systems—has introduced pollutants, chemicals, and hazardous waste into the environment. As many environmental health hazards are relatively new, the long-term effects on human health are yet to be understood. However, it has become increasingly clear that factors in the environment can affect health and that it is important to research and better understand their consequences.

While new technologies may introduce new hazards, research and regulations can also offer improved detection of and protection from such hazards. For example, transportation vehicles are a major source of air pollution in cities around the world. Through fuel combustion and evaporation, motor vehicles produce pollutants such as carbon monoxide, hydrocarbons, nitrogen oxides, and fine particles. Air pollution can irritate the eyes and throat, aggravate existing medical conditions such as asthma, and lead to heart and lung disease. As the harmful effects of air pollution have become more evident, steps have been taken to reduce the amount of pollution that people are exposed to, for example, through better engine designs and cleaner-burning gasoline formulations as well as emission standards and laws.

Another pollutant that has been controlled is lead, a toxic metal that was once commonly used in many products including gasoline, solder, plumbing, paints, ceramic cookware, and cosmetics. However, in the mid-twentieth century, research indicated that there were serious health risks from even low-level lead exposure; elevated levels of lead in the blood causes illness, can seriously affect mental and physical development in children, and can be fatal. Since then, leaded gasoline has been banned, lead-based paints are no longer used in homes, and older water pipes have been replaced. In addition, education and awareness of the problem are part of the solution. For example, in recent years there has been a concerted effort to reduce the frequency of childhood lead poisoning by keeping children away from deteriorating lead paint and lead dust and promoting routine medical testing. As a result, the incidence of elevated childhood blood lead levels has decreased.

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