There are many factors that can lead to poor air quality, but the two most common are related to elevated concentrations of ground-level ozone or particulate matter. Ground-level ozone forms when nitrogen oxides from sources like vehicle exhaust and industrial emissions react with organic compounds in the presence of heat and sunlight. Particulate matter refers to a mix of very tiny solid and liquid particles that are in the air we breathe. It can come from sources like vehicle exhaust, wildfires, and industrial emissions. Other factors that can affect air quality include weather conditions such as sunshine, rain, higher temperatures, wind speed, air turbulence, and mixing depths. Warmer temperatures and sunlight may also hasten the creation of ground-level ozone and smog, while warmer weather may lengthen the summer smog season. Air pollution can also come from burning fossil fuels, which releases gases and chemicals into the air. Pollutants of major public health concern include particulate matter, carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide.