San Joaquin Valley air quality explained – by the numbers

According to information in an Apr. 11, 2019 press release, the American Lung Association is soon to publicly release its much anticipated 2019 State of the Air report that, presumably, will once again confirm what many already know: that California’s San Joaquin Valley has America’s worst air quality where fine particulate matter pollution is concerned and is among those places in the nation with the worst ozone.

But, it isn’t this year’s State of the Air report that is being examined here in detail. Rather, it is the San Joaquin Valley Air Pollution Control District’s (Valley Air District) annual Report to the Community 2018-2019 installment (Report) that is being referenced here instead.

California’s expansive and agriculturally fertile yet often air-polluted San Joaquin Valley

San Joaquin Valley air and other basics

Immediately, it is helpful to note that California’s approximately 24,000-square-mile San Joaquin Valley basically extending from Stockton (San Joaquin County) in the north to Bakersfield (Kern County) in the south and situated between the Sierra Nevada and Coast mountain ranges to the east and west, respectively, with roughly 4 million residents, has long had – and has had to deal with the effects of – air pollution. The Valley’s fertile soils, meanwhile, support one of the region’s chief industries and activities – agriculture.

That said, Valley air quality over the years has improved. The numbers bear this out as will be seen below. That having been said, neither is the Valley’s air clean nor does it meet current health standards for either 8-hour ozone or daily or annual fine particulate matter – PM 2.5 (particulate matter less than 2.5 micrometers in aerodynamic diameter – roughly one-thirtieth the diameter of a strand of human hair). Studies indicate that both air-pollution types contribute to a host of health-related issues including coughing, wheezing, asthma, chronic obstructive pulmonary disease (COPD), respiratory and cardiovascular disease, lung cancer and premature death among others like pressure on the sinuses, pain or tightness in the chest as well as breathing difficulties.

From the Air Quality MattersWill San Joaquin Valley meet EPA’s newest ozone standard by 2037? – 3” post of Jan. 9, 2016, under the “California and San Joaquin Valley particulars” subsection, what is important to take away is that Valley population grew at an average annual rate of 2.035 percent between 1970 and 1990 compared to an average annual 1.645 percent population growth rate for California as a whole. Translated, this means that while the Valley’s population represented 8.15 percent of the state’s total in 1970, California’s population percentage relative to that of the Valley’s had shrunk by 2.85 percent by 2015, the Valley’s percentage thus increasing to 11 percent meaning that that kind of growth puts additional pressure on the Valley (its residents, businesses and industry) to reduce air pollution. This means reducing air pollution in the Valley will be that much more difficult a proposition.

Detailed SJV air quality data

In the San Joaquin Valley mobile sources contribute the most to air pollution here. This source is responsible for contributing 85 percent of all Valley air pollution with stationary sources accounting for the rest or 15 percent, this according to information presented in the Report. Literally making air-cleanup matters more of a burden, are geographical features, weather conditions and other factors.

Again referring to the Valley Air District’s 2018-’19 Report, spread across pages 3 (with detailed ozone data presented) and 4 (with detailed PM 2.5 data presented) are trends for both Valley ozone and PM 2.5 going back as far as 2002. Incidentally, in the case of one graph, “Decrease in 8-Hour Ozone Exceedances,” (on the Report‘s page 3) data goes back as far as 1992.


Ozone is a corrosive, colorless gas, and with regard to “County Days Over Federal 8-Hour Ozone Standard,” the trends for all three standards (i.e., the 1997 standard of 84 parts per billion (ppb), the 2008 standard of 75 ppb and the 2015 standard of 70 ppb), generally speaking, have been negative, this over a period of 17 years from 2002 to 2018. The same holds true regarding the “Decrease in 8-Hour Ozone Exceedances” Valley-wide between 1992 and 2018 as it relates to the 1997 and 2008 standards. Similar information as it has to do with the 2015 standard was not presented, apparently. Meanwhile, for “Ozone Season Good and Unhealthy AQI [Air Quality Index] (May-Sep)” readings, from 2002 to 2018, the trend in “Unhealthy AQI Days” has overall been favorable.

As for “Good AQI Days,” there is quite pronounced variability (there is a frequent switching of direction). For example, “Good AQI Days” ranged from about 345 in 2005 to around 155 in 2006, rising to roughly 335 in 2007 and retreating to 180 in 2008, perhaps influenced to some degree by either economic or climatic conditions or both. A “Good AQI Days” high of right around 375 was attained in 2010 then fell to about 210 in 2011, climbed to almost 300 in 2013, dropping to just over 250 the following year, and have fluctuated between 250 and 300 from 2014 to 2018.

As for the Valley meeting standards for ozone, only the national 1-hour standard of 120 ppb, set on Feb. 8, 1979 (by the U.S. Environmental Protection Agency), has been reached. There is still some work to be done in terms of the Valley meeting the 1997 standard of 84 ppb, the 2008 standard of 75 ppb and the 2015 standard of 70 ppb, all measured over a period of 8 hours. In terms of improvement, as explained in the Report, Valley ozone levels need to drop six points in order for the region to be in attainment of the 1997 standard.1

Fine-particle pollution

PM 2.5 in the Valley has seen improvement as well.

For instance, for “Annual PM2.5 Design Value Trend,” the trend between 2002 and 2018 has been up and down, but overall, it has been negative, going from a high in 2002 of 90 micrograms per cubic meter of air to a low of about 58 micrograms per cubic meter in 2017. However, regarding “24-Hour PM2.5 Design Value Trend” for the same time period, ever since 2004, although there has been variation, the trend, relatively speaking, has been holding fairly steady, for the most part basically fluctuating between 58 and 78 micrograms per cubic meter of air. It should be noted that the 1997 annual and daily (24-hour) standards for PM 2.5 are 15 and 65 micrograms per cubic meter, respectively. Meanwhile, the EPA established the standards for PM 2.5 of 12 (annual) and 35 (24-hour) micrograms per cubic meter, respectively, in 2012 and 2006.

For “County Days Over 24-Hour PM2.5 Standards (Nov-Feb),” as it has to do with the 2006 standard of 35 micrograms per cubic meter, the trend line has been negative.

To put things in better perspective, days over standards numbered approximately 390 during the 2002-’03 season, bottoming out at about 90 during 2016-’17.

Of particular note is the relatively steep decline during the period of the Great Recession, roughly between the years of 2008 and 2011.

As for the 1997 standard of 65 micrograms per cubic meter, it’s a different story in the sense that, although there has been some variability over the years, it has been less pronounced compared to that related to the 2006 standard. Though still a “saw-tooth” pattern, the variation has ranged between a high of about 75 during the 2013-’14 season, zeroing out in 2003-’04, 2012-’13, 2015-’16 and again in 2016-’17.

With respect to “PM2.5 Season Good and Unhealthy AQI (Nov-Feb)” data, as to the number of “Good AQI Days,” overall the trend line from 2002 to 2018 has been positive. Meanwhile, such is not quite the same for “Unhealthy AQI Days.” It has been holding fairly steady, reaching a high of roughly 140 during the 2013-’14 season and bottoming all the way to zero in 2015-’16. No doubt on account of wildland fires burning throughout much of the state, in 2017-’18, said numbers saw a sharp climb. As would be expected the number of “Good AQI Days” during this same period, conversely, saw dramatic decline.

When ozone and PM 2.5 reach their peak

One last note, whereas ozone typically becomes problematic during warm-weather months, exceedances of PM2.5 pollution can happen anytime of the year, though high concentrations in the air are more common during the cold-weather months, that, of course, being associated with heightened residentially-based wood-burning activity.

For the full Report to the Community 2018-’19 issue from the Valley Air District, go here.


  1. Report to the Community 2018-’19, San Joaquin Valley Air Pollution Control District, “Valley Experiences Another Great Ozone Season,” p. 7

This post was last revised on Dec. 23, 2020 @ 2:03 p.m. Pacific Standard Time.

Published by Alan Kandel