Air Pollution 101

As one of the top-ranked areas for ozone, particle pollution, and toxic air releases, the Houston region is no stranger to air quality concerns. On this page, you’ll find information about air pollution and what you can do to reduce your exposure, as well as links to further resources.

What is Air Pollution?

An air pollutant is a substance in the air that can have adverse effects on humans and the environment. Pollutants can take the form of gases, solid particles, or liquid droplets and can be classified as primary or secondary:

Primary Pollutants

Primary pollutants are substances emitted directly from a source, such as the carbon monoxide gas from a motor vehicle exhaust or sulfur dioxide released from factories.

Secondary Pollutants

Secondary pollutants form when primary pollutants react in the atmosphere. For example, ground-level ozone is created when hydrocarbons (HC) and nitrogen oxides (NOx) combine in sunlight.

Some pollutants may be both primary and secondary. Formaldehyde, for instance, may be emitted directly from a source and may also be formed by secondary reactions of certain hydrocarbons.

Common Air Pollutants

Particulate matter (PM) is a mixture of solid particles and liquid droplets. PM can cause a variety of health effects, including cardiovascular disease, stroke, nervous system impairment, respiratory illnesses, and even cancer depending on the chemical and physical characteristics of the particles.

The U.S. Clean Air Act sets standards for two types of particulate matter: coarse particulate matter (PM10), with a diameter of 10 microns (a micron is one-millionth of a meter), and fine particulate matter (PM2.5), with a diameter of 2.5 microns. Examples of PM10 include dust, pollen, and mold. Examples of PM2.5 include particles of different metals and smoke from diesel engines. In general, finer particles (PM2.5 and smaller) pose the greatest risk to human health. 

While particulate pollution may be directly emitted from many common sources such as fires, construction sites, and roadways, particulates are often formed secondarily as the result of chemical reactions from industrial emissions, automobile emissions, and other combustion processes.

Visit the U.S. EPA website to learn more about PM.

Ground-level ozone, or smog, is a dangerous pollutant that poses a serious threat to human health by irritating our lungs. For the most part, ozone is not emitted directly into the air but is created through a chemical reaction: Nitrogen oxides (NOx) and volatile organic compounds (VOCs) from cars, refineries and other sources combine in the air and are heated by the sun to form ozone. 

Visit the U.S. EPA website to learn more about O3.

Nitrogen oxides (NOx) include nitrogen dioxide (NO2) and nitric oxide (NO) – highly reactive compounds that are formed by the combustion of fossil fuels as well as some natural processes. The most problematic sources of NOx emissions are a result of the reaction of atmospheric nitrogen and oxygen that occurs in the high temperature conditions of fossil fuel vehicle engines, power plants and industries. Of this group of gases, nitrogen dioxide (NO2) is of the greatest concern. 

Of primary concern to the Houston region is the role NO2 plays in the formation of ground-level ozone. NO2, particulate matter pollution (PM), and volatile organic compound (VOC) emissions react with sunlight and contribute to the formation of ground-level ozone throughout the eight-county metropolitan area.

As a primary pollutant, high concentrations of NO2 can lead to a variety of adverse health effects on the respiratory system, including reduced lung function and growth. Increased levels of NO2 may have significant impacts on people with asthma because it can cause more frequent and more intense attacks. Children with asthma and older people with heart disease are most at risk.

Visit the U.S. EPA website to learn more about NO2.

Sulfur dioxide (SO2) is highly reactive, colorless gas with a pungent smell. It is primarily produced by the burning of fossil fuels and the smelting of mineral ores containing sulphur.

Exposure to SO2 harms the respiratory system, causes irritation of the eyes, and can exacerbate cardiovascular diseases. In addition, SO2 can contribute to the formation of secondary pollutants once released into the air, including particulate matter and acid rain.

Visit the U.S. EPA website to learn more about SO2.

Air toxics, or hazardous air pollutants, are chemical pollutants that may cause cancer or other serious health effects in humans or may cause environmental or ecological damage. Most air toxics are a result of human activity. High concentrations of air toxic emission sources are found in many Houston-area industrial facilities. Other sources include fuels, engine exhaust, and consumer products such as building and cleaning materials. These chemicals not only contaminate the air, but may also affect the food chain, water quality, contaminate soils, and cause property damage.

Unfortunately, because Houston is home to one of the largest concentrations of petrochemical facilities in the world, the Houston metropolitan area is also home to some of the highest air toxic emission release rates in the United States. Of 187 hazardous air pollutants (HAPs) regulated by the U.S. Clean Air Act, 161 are present in varying concentrations at different locations in the Houston area, and, in 2017, nearly 17 million pounds of toxic air pollution were emitted by industrial sources in the Houston metropolitan area.

Communities that are most at risk of developing health effects from air toxics are the neighborhoods located in close proximity to petrochemical plants and refineries. These areas are called fenceline communities because they are literally in the backyard of some major industrial facilities. 

Visit the U.S. EPA website to learn more about air toxics.

Carbon monoxide (CO) is a colorless and odorless gas which is formed by the incomplete burning of carbon-based fuels. Outside, major sources include motor vehicle exhaust and machinery that burn fossil fuels. Indoors, CO is usually generated by malfunctioning or misused fuel-burning appliances and contained in cigarette smoke.

Exposure to CO can cause immediate health problems by preventing enough oxygen from reaching the body’s organs (e.g. the heart and brain) and tissues. At extremely high levels, it can cause death (carbon monoxide poisoning). In addition, regular long-term exposure to low levels of CO has been linked with increased risk of a range of health problems, including potential neurological damage.Visit the U.S. EPA website to learn more about CO.

Learn more about Houston’s “Dirty Dozen” air pollutants in our factsheets


What Are the Sources of Air Pollution?

While outdoor air pollution originates from both natural and man-made sources, the contribution from human activities far exceeds natural sources. Often times indoor air pollution can be even worse than outdoor air pollution depending on the chemicals in your building.

Mobile sources are vehicles used in all modes of transport, including cars, trucks, trains, ships and planes. While fossil fuel combustion is the major culprit of mobile source air pollution, tire and brake pad wear is also a significant source of mobile emissions.

There are two types of stationary air pollution sources:

  • Major stationary sources are defined as sources that emit 10 tons per year of any of EPA controlled toxic air pollutants, or 25 tons per year of a mixture of air toxics. These sources include industrial facilities like chemical plants, steel mills, oil refineries, power plants, and hazardous waste incinerators.
  • Stationary area sources consist of smaller-size facilities (e.g. metal recycling facilities, concrete batch plants, dry cleaners, gas stations) that release lesser quantities of toxic pollutants into the air. Collectively the emissions from these sources can be of concern – especially in heavily populated areas. The category also includes agricultural areas.

Natural sources are naturally occurring events which cause air pollution and include dust storms, wildfires, and volcanic eruptions. The ocurrence of these events is becoming more common with climate change, which heightens their impact on air quality. 

Indoor sources include building materials and a range of household goods. Inadequate ventilation, high temperatures and humidity levels can increase indoor pollutant levels.

Health and Equity Effects

Worldwide, an estimated 4.2 million premature deaths globally are associated with poor outdoor air quality, mainly from heart disease, stroke, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections in children.

While everyone is affected, most of the disease burden is borne by marginalized populations who tend to live near busy roads and industrial sites with higher air pollution levels.

Other vulnerable groups include people with pre-existing lung or heart disease, as well as children and the elderly. Due to their developing lungs, children are particularly susceptible to the harmful impacts and face an increased risk of deficits in lung function and long-term reductions in lung growth rate.  

Improving air quality would reduce the many premature deaths linked to air pollution and lead to improved health and quality of life for all Americans. Reducing air emissions is also necessary for achieving health equity since cleaner air would bring large health benefits especially to those who currently bear the brunt of the exposure.

How to Stay Safe?

Until air pollution is stopped at source, minimize your exposure with these simple actions:

Air Pollution Regulation in the U.S. and Texas

The Clean Air ActThe Clean Air Act is a US federal law designed to protect human health and the environment from harmful air pollution caused by a diverse array of pollution sources. 

The Clean Air Act requires the EPA to issue minimum standards, known as National Ambient Air Quality Standards (NAAQS), for six common primary and secondary air pollutants known as criteria pollutants: ground level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide.

Other provisions of the Act include reducing emissions of hazardous or toxic air pollutants that pose cancer or other serious health risks; reducing concentrations of air pollutants that cause regional haze or acid rain; and phasing out production and use of chemicals that deplete the stratospheric ozone layer

The Texas Commission on Environmental Quality (TCEQ) is the agency assigned to enforce the Clean Air Act in Texas. However, the TCEQ’s enforcement record leaves much to be desired: A recent investigation by the Environmental Integrity Project found that the TCEQ issued penalties for less than 3 percent of unauthorized air pollution releases from 2011 to 2016.

While the Houston area has never met the air quality standards set for ozone, the Clean Air Act has driven air quality improvements nationally for nearly 50 years, preventing hundreds of thousands of cases of serious health effects each year. 

It is critical that the Act remains enforced to ensure continued protection of all Americans from harmful impacts of air pollution.

Read More About the Clean Air Act

What You Can Do

There are many ways you can help. Whether it’s educating others, adding your name to a petition, or reaching out to your leaders, even a small action can move us toward a healthier future.

Join our community of clean air advocates!

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