Carbon monoxide (CO) is a colorless, odorless gas that can be harmful when inhaled. It is released when something is burned.

SOURCES: CO primarily comes from cars, trucks, and other machinery that burn fossil fuels (oil, gasoline, coal, and natural gas).

HEALTH EFFECTS: CO reduces the amount of oxygen that can be transported in the bloodstream to critical organs, such as the heart and brain. When breathed in very high levels (typically in enclosed environments), CO can cause dizziness, confusion, unconsciousness, and death. Levels required for these effects are rare outdoors, but outdoor exposure can still be harmful for people with heart or lung problems, or people who are exercising.

ENVIRONMENTAL EFFECTS: CO is a significant contributor to ground-level ozone and smog.

Lead is a metal found in the earth’s crust that has some beneficial uses, but is toxic even in small amounts, especially to children.

SOURCES: It is emitted from a variety of human processes, such as metal processing, aircraft use of leaded fuel, waste incinerators, electric utilities, and lead-acid battery manufacturing. The highest concentrations are usually found near lead smelting facilities. There are also traces of lead inside houses with lead-based paint (especially if it is deteriorating), which was banned by the EPA in 1978.

Lead dust can be inhaled, and traces of lead can be ingested if a person drinks water that has passed through lead pipes, or eats or drinks with dishes or glasses containing lead.

HEALTH EFFECTS: Lead accumulates in the bones once it enters the body. It can have a range of negative health effects on the kidney function and nervous, reproductive, immune, and cardiovascular systems.

Lead is particularly dangerous to children. Their bodies absorb more lead than adults’, and are more sensitive to its damage. Lead exposure can result in behavioral problems, learning deficits, slowed growth, hearing issues and anemia. Lead exposure in pregnant women can result in similar effects for the fetus, and can increase the probability of miscarriage.

ENVIRONMENTAL EFFECTS: Lead in soil and water can result in decreased growth and reproduction in plants and animals, and can affect the nervous system of vertebrates.

SOURCES: Nitrogen dioxide primarily enters the air through the burning of fossil fuels from cars, trucks, buses, power plants, and industrial processes.

HEALTH EFFECTS: Exposure to high concentrations of nitrogen dioxide in the air can aggravate respiratory diseases such as asthma, and can lead to coughing or difficulty breathing. Nitrogen dioxide is also a component of particulate matter and ozone (smog).

ENVIRONMENTAL EFFECTS: Like sulfur dioxide, nitrogen dioxide is a component of acid rain.

Nitrogen dioxide and other forms of nitrogen are key components of nutrient pollution in water bodies (along with phosphorus). Nutrient pollution is a widespread problem in the United States. It occurs when an excess of nutrients enter water bodies and cause excessive growth of algae. This blocks sunlight that is needed for plants in the water to grow, which kills them. Their decay process uses oxygen from the water, which kills animals in the water food chain that rely on oxygen.

There is “good” and “bad” ozone. According to the EPA, “good ozone occurs naturally in the upper atmosphere, where it forms a protective layer that shields us from the sun’s harmful ultraviolet rays.” Click here to learn more about good (stratospheric) ozone. However, ozone at ground level is harmful.

SOURCES: Ground level ozone is created when pollutants from cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. It is most likely to reach unhealthy levels on hot, sunny days in urban areas, but it can still do so during colder months.

HEALTH EFFECTS: Ground-level ozone can cause coughing, sore throat, difficulty breathing deeply, and airway inflammation. It can make the lungs more susceptible to infection and aggravate lung diseases such as asthma, emphysema, and chronic bronchitis. Ozone is the main ingredient in smog.

ENVIRONMENTAL EFFECTS: For certain plants, ozone can reduce growth and increase risk of disease, risk of damage from insects, risk of damage from other pollutants, and risk of harm from severe weather.

Particulate matter (PM) is a mixture of solid particles and liquid droplets in the air. Examples include dust, dirt, soot, and smoke, among others. They are classified by size. PM 10 are particles that are 10 micrometers wide and smaller, while PM 2.5 are even finer particles that are 2.5 micrometers wide and smaller (about 30 times smaller than the diameter of one human hair).

SOURCES: Particulate matter can originate from single sources such as construction sites, unpaved roads, smokestacks, power plants, industrial sites, and cars, or can be created by complex reactions of chemicals emitted from multiple sources.

HEALTH EFFECTS: Due to its small size, PM can be inhaled, can penetrate deep into the lungs or even the bloodstream, and can cause serious health issues. PM 2.5 particles have the greatest health risks. These risks include premature death in people with heart and lung disease, heart attacks, irregular heartbeat, aggravated asthma, decreased lung function and airway irritation, coughing, or difficulty breathing.

ENVIRONMENTAL EFFECTS: PM can be carried long distances by wind and can make water bodies acidic and change their nutrient balance, deplete soil nutrients, damage forests and farm crops, and contribute to acid rain (see Sulfur Dioxide section below for an explanation of acid rain).

SOURCES: Sulfur dioxide is both released naturally by volcanic activity and produced as a byproduct of copper extraction and the burning of fossil fuels. The largest sources are power plants and other industrial facilities. Smaller sources include metal smelting and cars and ships that burn fuel containing sulfur.

HEALTH EFFECTS: Exposure to sulfur dioxide can harm the respiratory system, make breathing difficult, and aggravate asthma. Sulfur dioxide is a component of particulate matter.

ENVIRONMENTAL EFFECTS: High concentrations of sulfur dioxide can decrease tree and plant growth. It also contributes to acid rain, which can be fatal to species in the water and on land that cannot tolerate acidic conditions. For example, acidic water can both kill adult fish and prevent fish eggs from hatching, thus killing off fish in that water body. On land, it can kill forests by causing aluminum to be released from the soil.