Qualité de l’air à Walnut Creek

What is the air quality index in Walnut Creek, CA?

In 2019, Walnut Creek’s average annual air quality met the US Environmental Protection Agency (EPA) qualification for “good” with a US AQI of 27, indicating that the air generally poses little risk to health. The city’s “good” status, however, oversimplifies the day-to-day situation by smoothing over short-term pollution events.

Since 2016, Walnut Creek air quality has failed to meet federal 24-hour PM2.5 targets, a measure which tallies the number of “unhealthy” PM2.5 days in a year.¹ There was an average of 11 unhealthy PM2.5 days a year in Walnut Creek from 2016 to 2018, greatly surpassing the federal allowance of 3.2 unhealthy days.

These PM2.5 pollution swings primarily occurred during the winter months as a result of wood burning and pollution-trapping temperature inversions, or as a result of wildfires in the late summer and early fall. The frequency of these high-PM2.5 events have earned the greater San Jose-San Francisco-Oakland, CA area its ranking as 3rd in the nation for worst 24-hour particle pollution out of 217 surveyed metropolitan areas.

Walnut Creek’s most polluted months for PM2.5 in 2019 were November and December, with monthly concentration averages of 14.2 μg/m³ (“moderate”) and 12.4 μg/m³ (“moderate”) respectively, roughly 3 times the six-month average of April to September, 4.7 μg/m³.

For ozone pollution, Walnut Creek meets federal attainment targets, although levels are close to the EPA threshold. In 2016 to 2018, there was an average of 2.7 unhealthy ozone days a year. The EPA targets no more than 3.2 unhealthy ozone days a year.

As a secondary pollutant, ozone is not emitted directly from ground sources, but rather is formed in the atmosphere when nitrogen oxides (NO_x) and volatile organic compounds (VOCs) react in sunlight. Since temperatures of 84 degrees or warmer are generally required for a reaction and hotter temperatures accelerate its formation, environmental scientists expect climate change to pose challenges for maintaining low ozone levels in the future. In the last 50 years, summer temperatures in Northern California have increased by 2.5 degrees.²

Air pollution in excess of government standards can contribute to adverse health effects in the general public, though sensitive groups including children, the elderly and those with heart or lung disease, tend to be more acutely affected. There are an estimated 92,532 residents in Contra Costa County with asthma, 43,209 with COPD, 63,409 with cardiovascular disease, 259,791 under the age of 18, and 181,443 over the age of 65.

Where is Walnut Creek smoke coming from?

Walnut Creek’s smoky air quality conditions are often the result of wildfires burning in the Bay Area. Wildfires have become increasingly frequent and severe with the escalation of global climate change. Such effects have been clearly noted in California and the Bay Area, which had record-breaking burned acreage in 2017 and 2018 as a result of wildfires.² 2020 also appears on track to exceed these years, potentially smashing all previous records.

According to a study by the Lamont Doherty Earth Observatory at Columbia University, climate change has doubled the risk of extreme fire conditions in California by creating hotter and drier environments. The amount of burned acreage per year, moreover, has jumped eight-fold since 1970.

The onslaught of wildfires and burned acreage in 2020 resulted from a dry lightning storm, which ignited 367 fires in the area. As of late August, the 2020 “lightning siege” scorched more than 1.1 million acres in a little more than a week.³ The “LNU lightning complex” and “SCU Lightning Complex fires” were of particular threat to Walnut Creek air quality, driving evacuations in some areas.

The plumes of smoke have had a far-reaching effect on cities around the Bay Area, greatly impacting San Francisco air quality , San Jose air quality, and Sacramento air quality.

Is Walnut Creek, CA safe from wildfire smoke?

Due to their size, which often span up to tens of thousands of acres, wildfire impacts on air quality can be significant and far reaching.

Real-time air quality in Walnut Creek should be monitored often in the case of active wildfires in the area. Walnut Creek AQI readings above 100, orange “unhealthy for sensitive groups,” should be met with action, such as reducing outdoor activity and wearing an N95 mask.

People seldom think of indoor spaces as being affected by wildfire smoke. Researchers, however, estimate that indoor particle levels tend to be between 49% and 76% of the increases in outdoor particle levels during wildfires.⁴ To reduce your exposure to outdoor air pollution in Walnut Creek, follow the below wildfire protocol:

● Check current and forecast air quality levels in Walnut Creek at the top of this page. Follow the health recommendations for current conditions.

● When Walnut Creek AQI levels are above 100, close windows and doors, and seal door gaps and window cracks. Set air conditioning (HVAC) systems with fresh air intake to their recirculate mode.

● Use air purifiers or high-efficiency HVAC filters to remove fine particles from the air. Run air cleaning systems as often as possible, on the highest fan speed.

● Avoid strenuous outdoor activity to reduce the amount of smoke you inhale (e.g. walk, don't run). Wear an N95 pollution mask if possible.

● Evacuate when air quality levels become “very unhealthy” or "hazardous."

Is today a Spare the Air day in Walnut Creek?

Walnut Creek Spare the Air alerts are issued by The Bay Area Air Quality Management District (BAAQMD) when air quality is forecast to be unhealthy.⁵ The alerts are meant to serve as both a means for sharing valuable health information and encouraging the community to collectively reduce emissions to help drive down pollution levels.

Spare the Air alerts are typically called as a result of high ozone pollution in the summer and high fine particle pollution (PM2.5) in the winter. The seasonal differences in Walnut Creek’s air pollution composition are the result of changing weather patterns.

Warm weather above 84 degrees and abundant sunshine is required for ozone, a secondary pollutant, to form in the atmosphere. Motor vehicles, power plants, factories, and construction are a few of the sources responsible for releasing ozone precursors like nitrogen oxides (NO_x) and volatile organic compounds (VOCs). These react in sunlight to produce ground-level ozone, often referred to as “smog”.

Walnut Creek ozone season runs from April to October. During these months, there are roughly 15 days with temperatures over 80 degrees, where ozone may accumulate and reach unhealthy levels.⁶ When a Spare the Air alert is issued for ozone pollution, residents may experience short-term effects such as coughing and difficulty breathing. In the long term, ozone exposure can cause respiratory infections, permanent lung damage, cancer, and early death.

During the winter, colder temperatures prevent ozone formation. Through this season, Walnut Creek air quality suffers from another pollutant: fine particle pollution (PM2.5). PM2.5 pollution is released daily from various sources of combustion, such as fuel combustion, coal-burning, wood burning, and wildfires. From November through February, PM2.5 levels are frequently 2 to 3 times summer levels. Domestic wood-burning, estimated to contribute more than one-third of particulate pollution, and a pollution-trapping weather effect, temperature inversions, are to blame. On average, the vast majority of Spare the Air alerts are issued as a result of high PM2.5 levels, roughly 11 a year.

When a Spare the Air alert is in place, follow Walnut Creek’s live air quality data at the top of this page and take the recommended health precautions. Then, help reduce city-wide emissions by reducing your pollution footprint. The most impactful way residents can reduce their air pollution footprint is to drive less or use more fuel-efficient, low-emission transportation options. Motor vehicles tend to comprise 40 to 75 percent of air pollution emissions, contributing to elevated levels of black carbon, PM2.5, nitrogen oxides, and volatile organic compounds, among others.

How bad is Walnut Creek ozone pollution?

The American Lung Association (ALA) 2019 State of the Air report card rated Walnut Creek and greater Contra Costa County a “D” for ozone pollution. The rating comes from the city’s average 2.7 unhealthy ozone days it experiences annually, a tally close to the 3.2 unhealthy ozone day threshold.

While Walnut Creek is a suburban city with a relatively small population of 70,166, its location at the junction of highways from Sacramento and San Jose (I-680) and San Francisco/Oakland (SR-24) as well as its general close proximity to these much larger cities contributes to the prevalence of ozone precursor pollutants NO_x and VOCs in Walnut Creek.

Warming temperatures as a result of climate change create more ideal conditions for ozone formation, rising levels and prolonging the ozone season in Walnut Creek. Targeting vehicular emissions is among the most effective ways to reduce ozone levels in the long term despite a changing climate.

Article Resources

[1] American Lung Association. (2020). State of the air – 2020.
[2] Ray S, Miller B, and Jones J. (2020, August 25). California’s new normal: How the climate crisis is fueling wildfires and changing life in the Golden State. East Bay Times.
[3] Andrew F, and Knowles H. (2020, August 23). 1.1 million acres burned in nine days in California, as new lightning-ignited blazes forecast into Monday. The Washington Post.
[4] Environmental Protection Agency (EPA). (2020). Wildfires and indoor air quality (IAQ).
[5] Spare the Air. (2020). What is Spare the Air? Bay Area Air Quality Management District.
[6] Current Results. (2020). San Francisco Temperatures: Averages by Month.