You are here » Programs»Sustainable Environment»Air»External Air Quality Sites»Particulate Matter

Particulate Matter

Particulate matter is small solid particles or liquid droplets from smoke, dust, fly ash and condensing vapors. It can be suspended in the air for long periods of time. It is generated from paved and unpaved roads, woodsmoke, burning fuels, earth moving, mining, construction, vacant lots and agricultural activities.

These microscopic particles can affect breathing and respiration, cause lung damage and possibly cause premature death. Children, the elderly and people suffering from heart or lung disease are especially at risk.

The larger particles are mostly deposited in the nasal passages, while the very small particles can penetrate and be deposited in the lung sacs and membranes. Particulate matter can alter the body’s defense systems and cause cancer. Additionally, these particulates can damage paint, enhance metal corrosion, and soil buildings and clothing and reduce visibility. 

Before 1988, particulate matter was measured as total suspended particulates. The revision of the U.S. Environmental Protection Agency's health standard for particulates changed the emphasis to PM10, which are particles with a diameter less than or equal to 10 micrometers. Recent research shows that the smaller particles pose a greater threat than the larger particles. 

In 1997, the primary particulate matter standards were further revised, changing the form of the PM10 standard and creating a new 24-hour and annual PM2.5 standard. This was done to protect the public against the health effects associated with fine particles. The EPA revised the health standards for particulates in December 2012. It affected the annual PM2.5  standard only. The EPA decided that the 24-hour PM2.5 and the 24-hour PM10 standards, established in 2006,  were protective of human health and did not need revisions.

The EPA sets federal standards to protect public health and welfare. The limits for particulate matter are set at the following levels:


PM10 Standard

PM2.5 Standard

24-hour average

150 µg/m3

35 µg/m3

Annual average


12 µg/m3


Particulate Matter Trends

(click on graphs below to view the pdf version of the graph)

Short-term Trends:

In Pima County, the natural desert background comprises about one third of the typical particulate matter concentrations. Particulate concentrations tend to be higher in winter and increase during dry periods.

The graph below shows the monthly average trend of PM10 and PM2.5 concentrations at the Orange Grove monitoring site for 2012.

Long Term Trends:

PM10 levels have shown large annual fluctuations due to changing meteorological conditions and localized emissions around the monitoring sites. PM10 monitoring began in Pima County in 1988. Tucson violated the 24-hour PM10 standard after four exceedances were recorded in 1999. The exceedances were found to be due to high wind natural events, along with an extended period of low rainfall. The Pima Cuonty Department of Environmental Quality (PDEQ) has measures in place to protect public health and welfare from airborne dust.

PM2.5 levels over the 15-year monitoring period have been well below the federal health standards.

Effective programs in the Tucson area that help reduce particulate matter levels by promoting the use of alternate modes are:

Alternate modes such as carpooling, taking the bus, walking, and bicycling, and the use of telecommuting all serve to reduce the number of motor vehicles on the road.

For dust control, Title 17 of the Pima County Code outlines activity permits and performance standards required for construction activities.

For tips on how to keep our air healthy, visit or the Clean Air Starts With Me Facebook page.      


©Pima Association of Governments | 1 E Broadway Blvd, Suite 401, Tucson, AZ 85701 | Telephone (520) 792-1093, FAX (520) 620-6981