Riverside-San Bernardino-Ontario, CA
- High ozone days per year: 291
- No. of days with unhealthy particle pollution: 18
- People with asthma: 341,128
- Population: 4,580,670
With dangerously high ozone days nearly 80% of the year, the Riverside-San Bernardino-Ontario metro area in Southern California has the worst air pollution in the nation. The area’s climate and geography – hot and surrounded by mountains – play a huge role, as does the heavy truck traffic to and from the region’s many distribution warehouses.
