CO is a colorless, odorless gas that’s harmful in large amounts and is generally released when something is burning. According to the EPA, “Breathing air with a high concentration of CO reduces the amount of oxygen that can be transported in the blood stream to critical organs like the heart and brain. At very high levels, which are possible indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness, and death.” CO is commonly produced by fossil-fuel burning machinery, such as furnaces or unvented kerosene or gas space heaters.

Of the group of sulfur oxides, sulfur dioxide (SO₂) “is the component of the greatest concern,” says the EPA. The agency reports that the largest source of SO₂ is the burning of fossil fuels at power plants and industrial facilities, though smaller sources of emission include extracting metals from ore; vehicles, ships, and heavy equipment; and natural sources like volcanoes. “Short-term exposures to SO₂ can harm the human respiratory system and make breathing difficult,” the EPA says, and at high concentrations gaseous sulfur oxides can damage foliage and stunt growth.

Also called particle pollution, particulate matter is the term used for a mix of solid particles and liquid droplets in the air: “These particles come in many sizes and shapes and can be made up of hundreds of different chemicals,” the EPA explains. When particulate matter contains particles that are small enough, they can be drawn deeply into the lungs and even into the bloodstream. Because smaller particles pose the greater danger in this context, the EPA does not regulate particles larger than 10 micrometers; particles less than 2.5 micrometers, which are regulated, pose the most serious health risk.

Lead is a naturally occurring heavy metal; it’s dispersed into the air though ore and metal processing, piston-engine aircraft operating on leaded aviation fuel, waste incinerators, utility operation, and lead-acid battery manufacturing. According to the EPA, “Once taken into the body, lead distributes throughout the body in the blood and is accumulated in the bones.  Depending on the level of exposure, lead can adversely affect the nervous system, kidney function, immune system, reproductive and developmental systems, and the cardiovascular system.  Lead exposure also affects the oxygen carrying capacity of the blood.”

Nitrogen oxides include nitrous acid, nitric acid, and nitrogen dioxide, or NO₂, which is used as the indicator for the larger group of nitrogen oxides. NO₂ primarily enters the air through emissions from cars, trucks, busses, power plants, or off-road equipment. The EPA explains the health effects of NO₂: “Breathing air with a high concentration of NO₂ can irritate airways in the human respiratory system. Such exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing, or difficulty breathing), hospital admissions, and visits to emergency rooms.”

According to the EPA, Ozone is both good and bad “depending on where it is found.” We rely on ozone that occurs naturally in the upper atmosphere and forms a protective layer that shields the earth from ultraviolet rays from the sun. However, ozone at the ground level has a negative impact on people and the environment. Ground-level ozone is the result of chemical reactions between nitrogen oxides and volatile organic compounds. “This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight,” the EPA states. Breathing ozone can “trigger a variety of health problems including chest pain, coughing, throat irritation, and airway inflammation. It also can reduce lung function and harm lung tissue.”