The World Health Organization believes air pollution causes cancer in humans.

Smog. Vehicle exhaust. Factory smoke. Forest fires. Pollution gets into the air in many ways. The Environmental Protection Agency considers ozone and particulate matter to be the two most common types of air pollution. According to the American Lung Association, in 2013 California had the most problems with these contaminants, while the very cleanest cities were in Iowa, Wyoming and North Carolina. Though ozone and PM are widespread, other types of air pollution cause trouble throughout the United States.

A Bad Combination

Ozone is an unusual type of pollution because it’s not directly released into the environment. It’s created when two kinds of gases, hydrocarbon vapors and nitrogen oxides, combine in sunlight. Hydrocarbon vapors come from many sources, such as vehicle exhaust and fumes from factories, gas stations, paint and refineries. When fossil fuels are combusted in motor vehicles, trains, ships and power plants, nitrogen oxides are released. Ozone injures certain plants and trees and causes lung damage in humans. In addition, ozone is a component of smog, which contributes to respiratory problems and climate change.

Particle Problems

Soil, dust, acids, organic chemicals and metals make up particulate matter. Burning of any kind, including vehicles, factories and forest fires, releases PM into the air. In addition, road dust and gases from power plants distribute particles. The tiniest of these drops and particles are most dangerous to humans. This PM can make its way past protection in the nose and throat and go into the lungs. PM is particularly dangerous to the elderly and individuals with heart and lung diseases. Breathing becomes difficult, coughing and shortness of breath occur.

Secret Gas

Like many types of air pollution, carbon monoxide is created during the process of burning. The major producers of CO are motor vehicles, although it also comes from cigarette and factory smoke. Because CO is both odorless and colorless, it’s difficult to know when the gas is present. After being breathed in, CO enters the blood supply. It attaches itself to hemoglobin in the red blood cells, which normally carry oxygen throughout the body. With CO in the these cells, the blood distributes less oxygen. Without an adequate supply of oxygen, the body experiences symptoms such as tiredness, dizziness, difficulty breathing, headache and nausea. Too much CO leads to death.

Combustion Challenge

Burning fossil fuels releases sulfur dioxide into the air. Most of this comes from power plants, but factories, trains and ships are also sources of the contaminant. Even short-term exposure to sulfur dioxide causes breathing problems, especially in children, seniors and asthma sufferers. Along with nitrogen oxides, sulfur dioxide is one of the primary components of acid rain. Acid rain damages trees, soil, buildings, paint, statues and sculptures.

Hopeful Signs

Though still a source of pollution, lead levels in the air have decreased dramatically in the past few decades. Due to EPA regulations that prohibit lead in motor vehicle fuel, by 1999 lead pollution of the air dropped by 94 percent. Some aircraft still use leaded fuel. In addition, lead smelters, waste incinerators and metal processing factories release lead into the air. Once taken into the human body, lead can affect the nervous, immune, cardiovascular and reproductive systems. It contributes to high blood pressure and heart disease. Also, children with high lead levels often show neurological effects, such as learning difficulties and behavior problems.