Part of that brown cloud that seems to float above the valley is made up of Ozone. Ozone if formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) interact in sunlight.
Most of these constituents come from internal combustion engines like our cars & trucks and gas powered lawn equipment.
Breathing ozone can cause a variety of health problems such as chest pain, coughing, throat irritation, congestion, increased risk of asthma attacks and other respiratory issues.
Maricopa County is currently in compliance with federal ozone standards. Unfortunately, the Environmental Protection Agency (EPA) may be revising the standards downward which would cause Maricopa County to no longer be in compliance with the federal standard.
You can do your part by telecommuting when possible, carpooling, riding public transit, riding your bike or walking rather than driving, and reducing unnecessary car trips on high pollution advisory days.