Situated at the northern end of the San Joaquin Valley, Modesto shares many of the same pollution issues other valley cities face. One of the biggest culprits: Exhaust from California's Route 99, a major truck route for agricultural produce haulers.
But Modesto also gets emissions and industrial drift from the nearby San Francisco-Oakland metro area.
The American Lung Association's Bonnie Holmes-Gen said reducing particle pollution in Modesto has proven tough on a local level as well. Many area residents burn wood for heat in the winter and the area is prone to droughts. In July, August and September, virtually no rain falls in most years. Wildfires can break out and keep skies smoky for weeks.
