Part of that brown cloud that seems to float above the valley is made up of Ozone. Ozone if formed when nitrogen oxides and volatile organic compounds 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.
You can do your part by telecommuting when possible, carpooling, riding public transit, riding your bike, walking rather than driving, and reducing unnecessary car trips on high pollution advisory days.