In addition to the oxygen we need to survive, the air we breathe carries a number of tiny particles, invisible to the naked eye, that are able to enter our airways and travel deep into our lungs. Some of these particles can increase disease risk, including fine particulate matter (PM 2.5), sulfur dioxide, nitrogen dioxide, ozone, and radon. Air pollution occurs both indoors and outdoors. In fact, concentrations of pollutants indoors can be two to five times as high as the outdoor concentration.

Adequate intake of water is necessary to sustain life. Water, like air, includes many microscopic particles. Microorganisms – bacteria, protozoa, and viruses -- and chemicals exist in our water supplies. Before the creation of sanitation systems to clean water, disease and death from water-borne illnesses, such as cholera (a bacterial infection that can be deadly), were quite common. Today, the government requires public water suppliers to use disinfection systems and monitor water quality. These systems rid the water of many microorganisms and have been successful in decreasing the number of people contracting water-borne illnesses. Despite these efforts, certain microorganisms and chemicals still are found in our water. Also, people who get their drinking water from private wells do not have to follow the same rules as public water suppliers. Their water still may contain chemicals or microorganisms. The primary uses of water through which individuals may be exposed to chemicals and microorganisms include drinking, cooking, and bathing.

In the same way that air and water can contain toxins, soil also may contain hazardous substances. Radon, a known carcinogen or cancer-causing substance, is a naturally occurring gas that leaches out of rocky soil and into homes, causing indoor air pollution. Toxic substances dumped at hazardous waste sites can corrode their containers, spilling their contents into the surrounding ground. Soil often contains pesticides used either commercially or at private residences. These toxins then may be ingested or absorbed when people eat crops grown in contaminated soil or walk or play on contaminated ground. In this fashion, soil can allow potentially toxic substances to enter into the human body.

In addition to air and water, a third requirement for life is food. We cannot live without nourishment, and food provides yet another route for toxins to enter the body. Infectious microorganisms, such as salmonella, cause food-borne illnesses. Chemicals such as pesticide residues on fruits and vegetables can be ingested along with food. Also, research has shown that dietary habits, such as intake of saturated fats, cholesterol, fiber, and fruits and vegetables, have a direct effect on human health.