The Facts

Particulate matter (PM) is a mixture of solid particles and liquid droplets. PM has a variety of health effects, including cardiovascular disease; stroke; nervous system impairment; and respiratory illnesses including asthma, respiratory infections, and allergy symptoms. The 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.


Types of PM2.5

Fine particles come in a variety of shapes and compositions. There are many ways in whichPM Shapes fine particles end up in our air. Some are emitted directly into the atmosphere and have common names like soot, smoke, and dust. Others move from the ground into the air via a process called “entrainment,” as with road grit stirred up by vehicles. Still other fine particles actually form in the atmosphere via accretion or chemical reactions. Particles that find there way into the atmosphere are called “primary” PM, while those that form in the atmosphere are called “secondary.” Generally speaking, because the contribution of secondary PM is less well understood, efforts to control particulate matter focus on primary PM.

Some fine particles are a mix of many different types of pollutants or toxins. A typical fine particle can include hazardous heavy metals, air toxics, and various types of carbon (not to be confused with carbon dioxide, a well known greenhouse gas.

PM2.5 Regulation

The Clean Air Act includes particulate matter as one of six “criteria pollutants” for which it sets National Ambient Air Quality Standards (NAAQS). We tend to focus on PM2.5 for two reasons. First, PM2.5 is more dangerous, in part because fine particles are small enough to be inhaled deep into the lungs and to pass from there directly into the bloodstream. Second, Houston is closer to exceeding the NAAQS for PM2.5, meaning that the amount of fine PM in our air is a potential public health concern. The PM2.5 NAAQS includes a twenty-four hour standard of 35 micrograms per cubic meter (μg/m³) and an annual standard of 12 μg/m³. There are no areas in Texas that do not meet these standards for PM2.5, although Texas is 5th in the U.S. for premature deaths from PM. You can view a map of areas that do not meet the PM2.5 standard (so-called “nonattainment areas”) on the EPA’s webpage.

PM and “Exceptional Events”

Believe it or not, particulate matter in Houston can come from many places across the globe. Fine particles can be lifted high into the atmosphere and travel across continents, or even oceans. On December 22, 2013, a widespread dust event originating in Mexico affected much of Houston.

Mexican DustMystery Dust Satellite

Agricultural fires in Mexico are also a well-documented source of particulate matter pollution in Texas. As, of course, are domestic wildfires, as this photo of the October 2015 Hidden Pines fire in Central Texas demonstrates.

Hidden Pines Fire

Other dust events come from much farther away. Very fine dust from the Saharan Desert can travel across the Atlantic Ocean in the upper atmosphere. This is a well documented phenomenon that was even reported by Charles Darwin during his 1845 expedition on the Beagle.

African Dust

According to the Clean Air Act, certain pollution events that are atypical or beyond control can be flagged as “exceptional events” and removed from reported air quality data. In Texas, this flagging of exceptional events has been used to artificially reduce our pollution levels when reporting to the Environmental Protection Agency.

We do not agree with this tactic. People have to breathe pollution no matter where it originates. And natural events that have been documented for 170 years are certainly not “exceptional.” But the State of Texas relies on the removal of “exceptional events” from its air quality data in order to represent to the EPA that Texas complies with the Clean Air Act. We disagree with this and we have made comments supporting our position: Letter to EPA on Historical Background PM in Houston.

For the time being, Houston remains in attainment of the federal standard for PM2.5. This is in part due to the removal of data claimed as “exceptional events.”

PM Advance

The Houston region is very close to violating the PM2.5 standard of 12 μg/m³. The Environmental Protection Agency developed the Advance Program for voluntary participation by such regions. The Advance Program was created to give areas an opportunity to reduce pollution and stay in compliance with federal standards. The ozone Early Action Compact that some Texas cities joined years ago is a recent example.

In Houston, the Houston-Galveston Area Council led participation in the EPA’s PM Advance program.  In 2014, HGAC and it’s Regional Air Quality Planning Advisory Committee (RAQPAC) created a subcommittee (on which I served) to develop a PM Advance Path Forward document for submission to EPA. The document that resulted,  “PM2.5 Advance Path Forward”, did more to catalogue reductions already achieved and actions already taken than to commit the region to specific actions that would further reduce pollution. (You can view a summary of the PM Advance Path Forward from HGAC.) The document also did not quantify PM2.5 reductions expected to result from the plan.

Diesel Particulates

Diesel particulates make up a significant portion of our fine particle air pollution and areDirty Diesel classified as an air toxic. They can cause chronic disease and contribute to the risk of developing cancer at any concentration. In fact, they carry the highest additional cancer risk of any air toxic in the Houston area. The International Agency for Research on Cancer, a division of the World Health Organization, has classified diesel PM as a Group 1 human carcinogen.

The Zero Campaign

There is a nationwide campaign to end diesel pollution emissions known as the Zero Campaign. The Zero Campaign was launched by the Moving Forward Network, a group of which Air Alliance Houston is a member and whose mission is “To transform the global trade system by supporting the organizing, advocacy, education and research efforts of partners around the United States toward improving public health, quality of life, environmental integrity, labor conditions and environmental justice.”

Currently, the Zero Campaign is collection signatures on a petition to tell Gina McCarthy and the EPA to END deadly diesel emission now. You can follow that campaign at #ZeroEmissionsNow.

Galena Park and Clinton Drive

Of the ten fine PM monitoring sites in Houston, the Clinton Drive monitoring site has perhaps the highest PM2.5 levels. This is due in part to Clinton Drive’s status as a major freight corridor for the Port of Houston. Air Alliance Houston, along with other nonprofit groups such as Environmental Defense Fund, has lead the effort to address the PM2.5 problem in the Clinton Drive area.

After extensive monitoring for fine PM2.5 in the town of Galena Park north of Clinton Drive, Air Alliance Houston concluded that community is likely exposed to ambient concentrations of fine particulates above the federal standard. You can read the full report of our work in Galena Park here. We are currently undertaking a similar study of PM2.5 in Pasadena. The results of that study will be published in 2017.

Particulate Matter Levels near You

There are approximately ten PM2.5 monitoring sites in the Houston region managed by local, state, and federal governments. Data from those sites is evaluated for compliance with clean air laws and it made available to the public. If you ever need assistance accesses or interpreting air quality data, please don’t hesitate to contact us!


The Texas Commission on Environmental Quality maintains a map of monitored regions throughout the state.

Clicking on a region on the map takes you to hourly data from active PM monitors in the region. Remember that the current federal standards for PM2.5 are 35 μg/m³ over a twenty-four hour period and 15 μg/m³ over a one year period.

EPA Data

The Environmental Protection Agency makes air monitoring data available to the pubic through its Air Data webpage. The reports and visualizations available on this page can be complicated. Again, please contact us!