Particulate matter consists of tiny dust particles, other solid particles (e.g., carbon) and water vapor that are suspended in the air. Although individual particles cannot be seen by the naked eye, when particulate matter levels are high they appear to us as smog, haze or brown cloud. Particulate matter concentrations are highest during the winter season when particulate emissions are trapped and kept close to the ground by thermal inversions. Thermal inversions cause air to stagnate. There are federal regulations governing the concentrations of PM-10 (particulate matter of 10 microns diameter in size) and PM-2.5 (2.5 microns diameter in size). The Maricopa County region has not attained the EPA standards for PM-10. The central portions of metropolitan Phoenix usually have the highest concentrations of particulate matter. Areas located close to construction sites disturbed vacant land, and agricultural activity also may have relatively high particulate matter levels.  In order to improve air quality, the cities, county, and state have adopted stringent air quality requirements.  To learn more about the air quality requirements in the city of Glendale Fugitive Dust Ordinance, click here.  Go to the Maricopa County website for information on the current air quality status and county requirements.