Qualité de l’air à Santa Clara County

How is the air quality in Santa Clara?

Santa Clara is a coastal city located 45 miles southeast of San Francisco, bordered by San Jose to the east and Sunnyvale and Cupertino to the west. As defined by the Environmental Protection Agency’s (EPA’s) Air Quality Index (AQI), Santa Clara’s air quality status is generally rated “good,” faring relatively well globally and in comparison to other cities across the United States. However, Santa Clara air quality conditions can reach levels classified as “unhealthy for sensitive groups.”

Santa Clara experiences periods of unhealthy air quality primarily due to ground-level ozone and fine particulate matter (PM2.5). According to the American Lung Association’s State of the Air Report, Santa Clara averages 3 unhealthy air quality days for ozone and 10 unhealthy days for PM2.5 annually (based on data collected from the 2016-2018 monitoring period). As a result, the city received grades of “D” and “F” for these measures, respectively. Santa Clara is also part of the San Jose-San Francisco-Oakland metropolitan area, which the State of the Air report ranked third among “U.S. Cities Most Polluted by Short Term Particle Pollution” and eighth among the “Most Ozone-Polluted Cities.”¹

The EPA has also designated Santa Clara County as being in “marginal nonattainment” for ground-level ozone. With 8-hour average concentrations of 73 parts per billion (ppb), Santa Clara County exceeds the federal National Ambient Air Quality Standard (NAAQS) of 70 ppb.² Santa Clara is also in federal nonattainment for 24-hour PM2.5, with a daily average value over three consecutive years of 48 micrograms per cubic meter (μg/m³), exceeding the NAAQS of 35 μg/m³. However, Santa Clara PM2.5 is currently rated as “good” for annual average concentrations of less than 12 μg/m³.³

Ozone and PM2.5 are both considered harmful pollutants that can cause numerous systemic symptoms in the human body when breathed in. Both have been shown to aggravate respiratory and cardiovascular diseases, reduce lung function and lung growth in children, increase coughing and chest discomfort, and shorten life expectancy.⁴

What causes air pollution in Santa Clara?

Santa Clara air pollution levels are affected by:

• climate
• weather patterns
• population density
• proximity to other cities
• geographic features

Santa Clara ozone is primarily a summer problem, occasionally reaching AQI levels between 151 and 200 (“unhealthy”). PM2.5 peaks in the winter and can also reach “unhealthy” levels during this period, typically in December and January.⁵

Santa Clara is located roughly 10 miles from San Jose, the “capital” of Silicon Valley and the largest municipality in the Bay Area both in terms of land size and population. As a global center for technology, over one million commuters travel to and from work in Santa Clara County each day, producing significant daily traffic congestion.⁶ Vehicle emissions, including volatile organic compounds (VOCs), carbon monoxide (CO), PM2.5, and PM10, are a significant source of Santa Clara air pollution as well as regional air pollution in the Bay Area.

Santa Clara’s geography, climate, and meteorological conditions affect pollution dispersion and subsequently make it more likely that pollutants accumulate near ground-level, until weather allows for normal pollution dispersion.⁷ Some examples of these factors include:

• Pacific High-Pressure Cell: a subtropical anti-cyclone and controlling climatic factor of the San Francisco Bay Area Air Basin (SFBAAB) that occurs during the summer.⁸
• Temperature inversions: form when hot air descends over cool coastal air in the high-pressure cell, acting as a lid that inhibits vertical air movement and traps pollutants.
• Mountains and hills: located both east and west of the city, these geographic obstructions can further restrict air flow.

In the summer, decreased rainfall can exacerbate these conditions, as rain is the primary mechanism of removing particles from the atmosphere. Santa Clara PM2.5 concentrations tend to be even higher during the winter due to colder temperatures and calmer winds

Easterly winds originating in the Sacramento Valley Air Basin (SVAB) can make it possible for pollution, such as smoke from agricultural burning, to drift from Sacramento, Modesto, Fresno, and other cities to Santa Clara. These conditions, combined with increases in local wood burning in residential fireplaces and woodstoves, wildfires, and controlled burns, all significantly contribute to unhealthy Santa Clara air quality during the winter.

Is it a Spare the Air Day in Santa Clara?

Spare the Air alerts are issued by the Bay Area Air Quality Management District (BAAQMD) when Santa Clara air quality index (AQI) levels are forecast to be greater than “100”, indicating “unhealthy” or worse air. The program follows federal air quality index levels, with “orange,” “red,” “purple,” and “maroon” triggering Spare the Air alerts.⁹

When a Santa Clara Spare the Air alert is issued, residents are encouraged to:

• take proactive measures to protect their health, such as staying indoors and limiting strenuous activity
• reduce pollution emitting activities, such as driving and wood burning

Santa Clara Spare the Air alerts also occur when:

• 24-hour levels of PM2.5 exceed or are expected to exceed 35.4 μg/m³
• 8-hour ozone concentrations exceed or are expected to exceed 70 ppb (“unhealthy for sensitive groups” or more severe)

In the summer, Spare the Air alerts are more commonly a result of ozone pollution. In the winter, PM2.5 accumulations resulting from temperature inversions are the most common trigger.

Is air quality in Santa Clara improving?

Santa Clara air quality has improved over time largely due to increasingly strong federal and state regulations on emissions from cars and industrial sources. BAAQMD efforts, including the Spare the Air program, have also made an impact. Since 1999, the 3-year annual average for PM2.5 has improved from 14.2 μg/m³ to 10.9 μg/m³, averaging a 4 percent improvement per year. The number of days that PM2.5 exceeds federal 24-hour standards has also decreased annually, from 24 days between 1999 and 2001 to 8 days a year between 2016 and 2018.¹⁰ Santa Clara ozone levels have steadily decreased during this time as well, with days exceeding the 70 ppb standard falling from 12 days on average per year between 1999 and 2003 to 3 days on average in the past 5 years.

Shelter-in-place efforts resulting from the COVID-19 pandemic have had some silver linings in reducing transportation-related greenhouse gas and PM2.5 emissions, although the long-term effects on Santa Clara air pollution levels remain to be seen. Between January and May 2020, Silicon Valley freeway vehicle miles traveled (VMT) declined by 41 percent, while Santa Clara all-road VMT declined by 75 percent.¹¹ These reductions in vehicle travel are likely to have significantly reduced air pollution from vehicle exhaust during this period.

In the long term, climate change has the potential to reverse these improvements in air quality, as temperature projections show a warming trend across the Bay Area. With temperatures increasing and heat events becoming more common, greater demands will be placed on energy production, which may increase air pollution and greenhouse gas emissions from power plants. Drier conditions due to heat are also directly linked to a longer and more severe wildfire season, which can skew pollution trends.

What is Santa Clara doing to improve air quality?

BAAQMD has been an influential regional agency for managing and improving Santa Clara air quality. Since 1955, the BAAQMD has developed Clean Air Plans (CAPs) that provide a comprehensive vision for the entire Bay Region and guide local decision-making for air quality regulation.¹²

Most recently, BAAQMD has focused on transitioning Santa Clara and the Bay Area to a post-carbon economy to comply with Assembly Bill (AB) 32, the California Global Warming Solutions Act of 2006.¹³ Santa Clara has also adopted its own Climate Action Plan to address emissions and climate change consistent with AB32 and the BAAQMD CAP. According to the Santa Clara plan, the primary sources of greenhouse gases (GHGs) in the City are commercial-industrial (60%), followed by transportation (30%).¹

According to the City’s most recent Climate Action Plan Annual Report, Santa Clara air quality improvement efforts in the commercial-industrial sector are aimed at coal-free and renewable energy. The City has made steady progress since 2008, when coal made up about 23 percent of Silicon Valley Power’s (SVP) energy portfolio. The City’s energy mix now consists only of:¹⁵

• wind
• solar
• geothermal
• hydroelectric
• landfill gas
• natural gas.

To combat transportation-related emissions, Santa Clara has implemented BAAQMD GHG reduction best practices, including the implementation of Complete Streets policies that promote walking and bicycling as viable transportation options over driving.¹⁶

In addition, Santa Clara has worked to strengthen the coordination between land use and transportation strategies by linking the reduction of vehicle miles traveled (VMT) with the various land use patterns in its four transportation districts:

• North of Caltrain
• Downtown
• El Camino Real Corridor
• Stevens Creek Boulevard

Land use designations with greater potential for trip generation, such as “regional commercial” and “low-density office,” are assigned greater VMT reduction requirements. Between 2008 and 2016, the impacts of these policies were estimated to be a reduction of 48,311 MTCO²e, equal to over 119 million VMT.¹⁸ Santa Clara continues to monitor emission reductions program in annual reports to BAAQMD.

+ Article resources

[1] American Lung Association (ALA). (2020). State of the Air.
[2] Environmental Protection Agency (EPA). (2020). 8-Hour Ozone Nonattainment Area State/Area/County Report.
[3] Environmental Protection Agency (EPA). (2019). Air Trends. Air Quality Design Value Reports.
[4] Environmental Protection Agency (EPA). (2020). Health and environmental effects of particulate matter.
[5] Bay Area Air Quality Management District (BAAQMD). (2014). Improving air quality & health in Bay Area communities.
[6] U.S. Census Bureau. (2015). 2011-2015 5-year ACS commuting flows.
[7] Bay Area Air Quality Management District (BAAQMD). (2011).Trends in Bay Area ambient particulates.
[8] Riordan, et al. (2016). Climate change & Bay Area human health facts, figures and science-based Storylines. Climate Readiness Institute.
[9] BAAQMD. (2015). Spare the Air program FAQs.
[10] California Air Resources Board (CARB). (2019). Air quality trends summary.
[11] Joint Venture Silicon Valley. (2020). Transportation, emissions & air quality.
[12] Bay Area Air Quality Management District (BAAQMD). (2017). Clean Air Plan: Spare the Air, Cool the Climate.
[13] City of Santa Clara. (2018). Climate Action Plan 2018 Annual Report.
[14] Environmental Protection Agency (EPA). EPA greenhouse gas equivalencies calculator.
[15] County of Santa Clara. (2015). Community Health Existing Conditions Report.
[16] City of Santa Clara. (2013). Climate Action Plan.