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What is the air quality in Chico California?

Air pollution is a public health crisis in Chico, California, where air quality levels rank among the worst in the United States. The city’s air quality status comes despite its relatively small population and rural location in Northern California’s Butte County. A combination of emission sources as well as meteorological, topographic, and geographic conditions shape the city’s poor air quality status, classified by the federal government as “moderate nonattainment.”¹

Air quality can be examined by a number of parameters spanning numerous pollutants and government measures. In the below analysis, focus is placed on Chico’s overall air quality index score as a representation of long-term mean pollution levels as well as the frequency of short-term, “unhealthy” PM2.5 and ozone pollution episodes.

On the most basic level, Chico air quality can be summarized by its annual air quality index (AQI) score. The AQI uses a complex formula to translate concentrations of 6 criteria pollutants to a single score that easily conveys associated health risk to the public. The AQI uses a scale of 0 to 500 (though values above 500 are possible), where higher levels indicate higher health risks and only values below 50 are considered “good” (air that poses little to no health risk).

In 2019, Chico averaged an air quality index (AQI) score of 29 (“good”), indicating that air quality in Chico, California was considered healthy on a typical day that year. This was an improvement over recent years. In 2017 and 2018, Chico averaged an AQI of 52 and 63 respectively. This places both years in the “moderate” category, posing some health risks, especially to those sensitive to air pollution, including:

• children
• the elderly
• pregnant women
• those with pre-existing heart and lung disease

While “good” air quality was the average in 2019, pollution varies throughout the year on a daily and hourly basis, fluctuating from predominantly healthy levels to occasionally unhealthy levels.

In order to account for the frequency of unhealthy pollution days, the Environmental Protection Agency (EPA) sets a secondary standard for short-term, or 24-hour, pollution. The EPA targets an allowance of no more than 3.2 unhealthy ozone days and 3.2 unhealthy PM2.5 days per year averaged across a 3-year monitoring period. In the most recent 2016-2018 monitoring period, Chico far exceeded the federal allowance for both ozone pollution and PM2.5 pollution and was deemed nonattainment for both short-term pollution measures.

Ozone pollution, a key component of “smog,” is prone to daily and seasonal fluctuations as a result of changing temperatures. Unlike a majority of air pollutants that are emitted directly into the atmosphere from combustion, ground-level ozone is formed in the lower atmosphere when the presence of sunlight and heat (typically in excess of 84 degrees) forces ambient gases, such as nitrogen oxides and organic compounds predominantly from fossil fuel combustion in motor vehicles, to chemically react. Since sunlight and heat can force this chemical reaction, ozone levels are at their highest daily levels during the afternoon and their highest annual levels during the summer.

In Chico, average temperatures during the summer months of May through October are generally above 80 degrees, with May through August experiencing roughly 14 hours of sunlight per day.² During these months and daylight hours, ozone is more likely to reach unhealthy levels.

Between 2016 and 2018, Butte County averaged 17.5 unhealthy ozone days per year, roughly 5 times the federal limit.³ According to the 2020 State of the Air report published by the American Lung Association (ALA), Butte County has never met federal attainment levels for daily ozone and ranks 18th nationally for high ozone days out of 229 metropolitan areas.

Unlike ozone, which peaks in the summer, PM2.5 is a dangerous and prevalent pollutant that peaks in the winter.

PM2.5 pollution describes particulate pollution spanning a range of chemical makeups that measures 2.5 microns in diameter or smaller. PM2.5 often describes ash, soot, chemicals, dust, dirt, pollen, and pathogens. Due to the near-microscopic size of PM2.5 particles, they are more acutely dangerous to human health due to their ability to penetrate deep into the lungs, sometimes bypassing the blood barrier to cause far-reaching bodily harm.

Spikes in PM2.5 pollution in Chico are most commonly associated with smoke from wildfires or domestic wood burning. It is for this reason that PM2.5 often peaks in the fall and winter.

Particularly destructive wildfire seasons tend to result in far more unhealthy PM2.5 days annually. This was noted during 2017 and 2018, which both became record-breaking wildfires in California. During the 2016 to 2018 monitoring period, Butte County averaged a total of 9.3 unhealthy PM2.5 days per year, exceeding the federal target by nearly 3 times and causing Chico to rank 17th for 24-hour particle pollution out of 216 metropolitan areas.

In 2019, a relatively mild year for wildfires, the months of January (13.7 μg/m³, “moderate”), December (13.3 μg/m³, “moderate”), and November (10.5 μg/m³, “moderate”) averaged the highest PM2.5 levels. 2018 represented a particularly destructive wildfire season: that year, the three most polluted months were November (75.1 μg/m³, “unhealthy”), August (29.7 μg/m³, “moderate”) and July (16.4 μg/m³, “moderate”), respectively, highlighting the impact of wildfires in the late summer and fall.

Has air quality in Chico, California improved?

Air quality trends in Chico, California have wavered since recorded measurements began at the turn of the century. Long-term trends depict overall improvements, while short-term trends show a recent increase in PM2.5 and ozone pollution.

Ozone trends in Chico, CA

In its 20-year recorded history published by the ALA, Chico has never met federal attainment levels for ozone pollution.

When records began in 1996, Chico experienced rising ozone levels year after year until peaking in the 2001-2003 monitoring period, with an average of 53.8 unhealthy ozone days per year. This increase is likely attributable to a growing population dependent on motor vehicles for commuting. Following this year, ozone levels gradually fell as a result of more stringent vehicle emission standards to their lowest point in the 2013-2015 monitoring period, with an average of 8.2 unhealthy ozone days per year.

Since then, ozone levels have been on the rise once again. In the most recent 2016-2018 monitoring period, Chico averaged 17.5 unhealthy pollution days a year. Recent gains are attributable to a combination of increased vehicle emissions resulting from a growing population and economy as well as climate change, with increasingly warm seasonal temperatures extending the ozone season.

PM2.5 trends in Chico, CA

PM2.5 levels in Chico have gradually improved, falling nearly 50 percent in two decades from an annual PM2.5 concentration of 14.6 μg/m³ in 2000 to 7 μg/m³ in 2019.

However, recent devastating wildfire seasons in 2017, 2018, and 2020, have caused PM2.5 levels to jump from their lowest point in 2014-2016. This jump is particularly evident in the frequency of short-term unhealthy PM2.5 days, which increased from an average of 1.7 days in 2015-2017 to 9.3 days in 2016-2018.

While emission-reducing policies result in lower average PM2.5 levels overall, unpredictable and sporadic wildfires will likely pose the greatest threat to Chico PM2.5 levels in the future.

What are the main sources of air pollution in Chico?

Emission sources in Chico, California include vehicular emissions, agricultural dust, energy demands met with fossil fuels, household energy consumption, and wildfires as a prominent natural cause. The city’s location at the floor of the Sacramento Valley, near the foothills of the Cascade Range to the north and the Sierra Nevada range to the east and south, contributes to pollution-trapping conditions in which emissions accumulate in the lower atmosphere.

While air quality in Chico, with an annual PM2.5 average of 7 μg/m³, fared better than that of neighboring cities in 2019, air quality was relatively worse in Chico during 2018 (17.6 μg/m³) and 2017 (12.8 μg/m³) as a result of wildfires burning in close proximity to the city.

• Yuba City air quality: 2019 Average: 9.6 μg/m³; 2018 average: 18.2 μg/m³; 2017 average: 12.6 μg/m³.
• Sacramento air quality: 2019 Average: 8.7 μg/m³; 2018 average: 14.7 μg/m³; 2017 average: 11.5 μg/m³.
• Oroville air quality: 2019 Average: 7.2 μg/m³. No data recorded for 2018 or 2017.
• Redding air quality: 2019 Average: 6.4 μg/m³. No data recorded for 2018 or 2017.

When will air quality improve in Chico?

Chico air quality is dependent on both emissions and weather conditions. While emissions are the source of pollution in the city, weather determines how quickly emissions disperse.

During extreme pollution events like wildfires, pollution levels can change quickly and drastically. Follow real-time and forecast air quality levels in Chico at the top of this page to prepare for episodes of pollution.

When air quality in Chico is deemed or forecast to be “unhealthy” or worse, simple precautionary actions can help reduce pollution exposure. To limit health impacts:

• Avoid outdoor activity and/or reduce exertion outdoors.
• Close doors and windows in buildings and vehicles.
• Use N95, KN95, or P100 air pollution masks capable of filtering out fine particulate matter (PM2.5).
• Set air conditioners to recirculate setting.
• If available, run HVAC systems with fine particle filtration at high speed and/or run air purifiers, if available.

Where is the smoke coming from in Chico California?

Smoke in Chico is typically the result of wildfires burning in the late summer and fall (usually from July to October). In recent years, Chico has repeatedly made headlines for air quality deemed “unhealthy” and even “hazardous” as a result of wildfires burning across the state.

Environmental scientists attribute the rise in wildfires in recent years to anthropogenic climate change caused by the burning of fossil fuels and increase in atmospheric greenhouse gases.⁴ As regional temperatures warm and snow melts sooner, more water evaporates from soils and bodies of water, causing plants to suffer increased water loss and leading to an accumulation of dry and dead forest undergrowth. Combined with a rising population and acts of human negligence, fires can ignite and burn faster and for longer periods of time.

Since 2016, numerous destructive fires have burned near Chico, notably including the 2018 Camp Fire and the 2020 North Complex Fire, which included the nearby Bear Fire.⁵ The rise in fire activity has become the new normal in the region, and both local and state governments continue to grapple with how to deal with this growing challenge.

Use the IQAir air quality map of Chico, California to identify which fires are active in the area. The map displays active fire data layered with measured, real-time air quality data to reveal how wind direction and speed are likely to influence Chico’s air pollution levels.

+ Article Resources

[1] Butte Environmental Council. (2020). Environmental health.
[2] Weatherspark. (2020). Average weather in Chico.
[3] American Lung Association. (2020). State of the Air – 2020.
[4] Union of Concerned Scientists. (2020, March 11). The connection between climate change and wildfires.
[5] Smith E. (2020, September 10). Column: If California wants to know how to cope with the apocalypse, ask Butte County. Los Angeles Times.