These western states vary greatly in air pollution issues. Although California has led the nation in efforts to reduce air pollution over the past three decades, it remains the state with the most serious pollution problems. By contrast, Hawaii continues to rank as one of the states with the least ozone and particle pollution.

Arizona
Arizona’s air pollution problems occur in Maricopa County, home to Phoenix, which recorded fewer unhealthful days for ozone and particle pollution in 2002-2004. By contrast, Flagstaff ranked on the list of the cleanest cities for ozone pollution and short-term particle pollution, while Coconino County was listed as one of the cleanest counties for ozone pollution. Tucson also made the list of the cleanest cities for short-term and year-round particle pollution, ranking 5^th on the latter list. Pima County ranked 18^th on the list of cleanest counties for year-round particle exposure.

California
California cities and counties continue to dominate the list of places with the highest number of days with high air pollution. Los  Angeles-Long Beach-Riverside continues to sit atop two of the lists of most polluted cities and its counties, while Bakersfield moved into the spot of the most ozone-polluted city in the nation. Kern County moved to the top of the list of most polluted counties for ozone, but Riverside remained atop both particle pollution lists. Eight other metropolitan areas in California made the list of the worst cities for short-term particle pollution: Bakersfield at 2^nd; Fresno-Madera at 3^rd; Sacramento-Arden-Arcade-Truckee, CA-NV at 8^th; Visalia-Porterville at 11^th; Hanford-Corcoran at 13^th; San Diego-Carlsbad-San Marcos at 14^th; Modesto at 18^th; San Jose-San Francisco-Oakland tied for 20^th; and Merced at 23^rd. Most of these same cities rank highest in year-round particle pollution as well: Bakersfield at 2^nd; Visalia-Porterville at 4^th; Fresno-Madera at 5^th; and Merced tied at 18^th.

On the list of most ozone-polluted cities, seven California cities follow Bakersfield for the first time at the top: Los Angeles ranked at 2^nd; Visalia-Porterville at 3^rd; Fresno-Madera at 4^th; Merced at 5^th; Sacramento-Arden-Arcade-Truckee, CA-NV at 7^th; Hanford-Corcoran at 11^th; and Modesto at 13^th.

On the list of most polluted counties, California has 12 on the list of the worst for short-term particle pollution. Riverside County tops the list as the most polluted, followed by Kern County at 2^nd; Fresno at 3^rd; Los Angeles at 4^th; San Bernardino at 7^th; Orange at 9^th; Sacramento at 12^th; Tulare at 15^th; Kings at 17^th; San Diego at 18^th; Stanislaus at 23^rd; and Contra Costa at 25^th. Nine counties are on the list of most polluted year-round by particles: Riverside in 1^st place; San Bernardino, 2^nd; Los Angeles, 3^rd; Kern, 4^th; Tulare, 6^th; Fresno, 7^th; Kings, 9^th; Orange, 10^th; and Merced, tied for 22^nd. California also had good news about air quality as well. Two counties ranked on the list of the 25 cleanest counties yearround for particle pollution: Lake tied for 9^th and Inyo ranked 17^th.

Some of the same counties rank among the worst for ozone pollution as well, led by Kern County, which had more unhealthful days in 2002-2004 than it had in 2001-2003. Following Kern are these 14 counties: San Bernardino County, 2^nd; Riverside, 3^rd; Tulare, 4^th; Los Angeles, 5^th; Fresno, 6^th; Merced, 7^th; El Dorado, 9^th, Sacramento, 10^th, Nevada, 12^th; Ventura, 13^th; Mariposa, 14^th; Kings, 18^th; Placer, 19^th; and Stanislaus, 22^nd. Good news: Thirteen California counties placed on the list of cleanest counties in the nation for ozone: Colusa, Glenn, Lake, Marin, Mendocino, Monterey, Napa, Plumas, San Francisco, San Mateo, Santa Cruz, Siskiyou and Sonoma.

Six counties in California had fewer high ozone days and improved their grades. Several counties improved their short-term particle pollution grades by recording fewer unhealthful days. Three counties—Alameda, Imperial and Plumas—improved from an F to a D. In addition, Salinas ranked as part of a select group of the cleanest cities in the nation for all three categories of pollutants. Redding, Santa Barbara-Santa Maria-Goleta and San Luis Obispo-Paso Robles joined the cleanest cities lists for shortterm and year-round particle exposure, ranking 24^th and 25^th on the latter.

Hawaii
By contrast to California, Hawaii records almost no days of unhealthy levels of ozone pollution. There are a few days each year with unhealthy levels of particle pollution in Honolulu County, due to the annual fireworks display at the New Year celebration. Despite that, Honolulu County ranked 11^th cleanest county for yearround levels of particle pollution. Maui County ranked as the 5^th cleanest for yearround particle pollution. Honolulu County and Hawaii County are also two of the cleanest for ozone pollution. The City of Honolulu remains on the list of cleanest cities for both ozone and year-round levels of particle pollution.

Nevada
Clark County in Nevada records that state’s highest numbers of unhealthful ozone and particle pollution days. Reno-Sparks ranked as the 21^st cleanest city for yearround levels of particle pollution and on the list of cleanest cities for short-term particle exposure. Douglas County and White Pine County made the list of cleanest for ozone.