Qualité de l’air à Mountain View, Santa Clara County

How polluted is the air in Mountain View?

In 2019, Mountain View averaged an air quality index (AQI) score of 28, thereby meeting the US Environmental Protection Agency (EPA) qualification for “good.” Despite a healthy status overall, Mountain View air quality varies day to day, with pollution levels ranging from predominantly “good” ratings to the occasional “unhealthy” and “very unhealthy” ratings.

Daily, or short-term, pollution is evaluated by the number of days per year deemed unhealthy. The US EPA targets an annual allowance of no more than 3.2 unhealthy PM2.5 days and 3.2 unhealthy ozone days. Between 2016 and 2018, Mountain View averaged a total of 13 unhealthy air quality days per year, 10.2 of which were attributed to PM2.5 pollution (thereby deemed “nonattainment” for exceeding the 3.2 day limit), and 2.8 of which were attributed to ozone pollution (thereby meeting “attainment”).¹

Particle pollution, or PM2.5, is primarily to blame for Mountain View’s wavering air quality. PM2.5 levels tend to rise more prominently during the winter months as a result of domestic wood burning and temperature inversions (a pollution trapping weather event) as well as sporadically in the summer and fall as a result of wildfires.

In 2019, Mountain View’s most polluted months for PM2.5 pollution were November and January. These months averaged a PM2.5 concentration of 15.9 μg/m³ (“moderate”) and 13.2 μg/m³ (“moderate”) respectively, roughly double the annual monthly average of 6.7 μg/m³.

Neighboring cities also affected by wildfire smoke experienced similar trends. The nearest of these, including Palo Alto, Redwood City, and San Jose, tended to fare only slightly better on average than Mountain View:

• Fremont air quality: 2019 average: 7.5 μg/m³; 2019 Monthly high: November, 19.1 μg/m³
• San Francisco air quality: 2019 average: 7.1 μg/m³; 2019 Monthly high: November, 14.5 μg/m³
• Palo Alto air quality: 2019 average: 6.5 μg/m³; 2019 Monthly high: November, 15.9 μg/m³ (January also experienced “moderate” air quality, 13.4 μg/m³)
• San Jose air quality: 2019 average: 6.4 μg/m³; 2019 Monthly high: November, 15.9 μg/m³
• Redwood City air quality: 2019 average: 6.3 μg/m³; 2019 Monthly high: November, 15 μg/m³

Mountain View air quality was cleaner on average in comparison to air quality in Fremont and San Francisco, both considerably larger cities with more residents, industry, and traffic.

While short-term PM2.5 pollution is the only measure for which Santa Clara County failed to meet federal attainment, short-term ozone levels are also of concern. Ozone differs from PM2.5 in that it is a gas pollutant and is not emitted directly into the atmosphere, but rather is formed when primary pollutants react in the presence of sunlight and heat (temperatures over 84°F).

It is due to this latter attribute that ozone is not more prevalent in the Bay Area. Such warm conditions are relatively rare in Mountain View’s moderate climate. The warmest month, September, has an average high of 78°F.² While warmer temperatures do often occur, they are infrequent, and as a result, ozone levels in Mountain View often remain within ranges considered healthy. Still, Santa Clara County averages 2.8 unhealthy ozone days per year, and more often, shorter periods of time (hours) during the heat of the day in the summer can reach unhealthy levels.

What causes air pollution in Mountain View?

Mountain View is a relatively small city in the heart of the Silicon Valley, just 13 miles northwest of San Jose. Named for its view of the Santa Cruz Mountains, air quality in Mountain View is largely shaped by its geographic location as it relates to pollution-trapping geography and weather events as well as transboundary emissions from neighboring cities in addition to local emissions.

Simply stated, air pollution is the result of emissions. In Mountain View, common emission sources include:

• gas-powered motor engines, including cars, trucks, construction equipment, ships and various watercraft
• a steel mill
• demand for energy production through fossil fuels, such as manufacturing, industry and household energy consumption
• winter wood burning
• wildfires

While air pollution is generated by emissions, it is often weather and geographic features, such as large bodies of water and mountains, that affect pollution dispersion and thus largely dictate air quality levels.

Mountain View’s location on the southwestern shore of the San Francisco Bay makes the city prone to marine inversions, a weather event in which cool surface-level air, created by the Pacific Ocean, becomes trapped beneath a warmer air layer above.³ In the case of Mountain View, warm air typically flows over the Santa Cruz Mountains or Diablo Range, creating a “lid” over the cool surface level air, carried in by sea-breezes.

Marine inversions in Mountain View can cause air pollution to accumulate and linger in the lower atmosphere until weather conditions change, elevating city-wide pollution levels substantially.

Is Mountain View, California affected by the fires?

Despite decades of progress in cleaning the air, Mountain View has recently experienced stagnant or worsening air quality for measures of PM2.5 and ozone pollution. The increasingly familiar siege of wildfires in the summer and fall are largely to blame.

Wildfires generate massive amounts of smoke, a combination of ambient particles (PM2.5 and PM10) and gases (carbon monoxide, nitrogen dioxide, formaldehyde, and acetaldehyde) that can travel thousands of square miles. The vast nature of wildfires and the resulting smoke plumes have impacted air quality throughout California and the Bay Area perhaps most significantly.

Most concerning is not the occurrence of wildfires, which are natural and necessary to our ecosystems, but rather recent trends indicating that wildfires are becoming far more prevalent and severe as a result of climate change.

Predictive models project that, by 2050, climate-change will cause a 54 percent increase in average area burned by wildfires while also elevating summertime PM2.5 levels by 30 to 40 percent in America’s West.⁴

Nearly half of the 20 largest wildfires in California history have occurred since 2017.⁵ The August Complex Fire of 2020, ignited by a siege of lightning strikes roughly 140 miles from Mountain View, became the largest recorded wildfire in California history. Pollution levels in Mountain View and the greater Bay Area were elevated far beyond typical local ranges for weeks at a time.

While 2017, 2018, and 2020 were all record-breaking years for wildfires, 2019 was relatively mild. The impact of this mild season is clearly evident. Bay Area locations tended to experience half the PM2.5 levels in 2019 as compared to 2018. San Francisco, for example, averaged a PM2.5 concentration of 12.5 μg/m³ in 2018, compared to just 7.1 μg/m³ in 2019. San Jose, likewise, experienced a PM2.5 concentration of 12.4 μg/m³ in 2018 and 6.4 μg/m³ in 2019.

Breathing wildfire smoke for prolonged periods of time can cause respiratory irritation and difficulty breathing as well as more acute long-term effects, including reduced lung function, inflammation, bronchitis, heart disease, and early death. Adverse health effects are often especially dire in children, the elderly, and those pre-existing heart and lung conditions.

During California’s wildfire season spanning July through November, take care to follow real-time air quality data in Mountain View and follow advised health recommendations.

Is air quality in Mountain View getting worse?

While Mountain View air quality has greatly improved in the last three decades, recent trends show stagnant or worsening air pollution levels for ozone and PM2.5 metrics.

While the frequency of unhealthy ozone days has decreased by 84 percent from 20 unhealthy days per year between 1996 and 1998 to 3.2 unhealthy days per year between 2006 and 2008, ozone has been on the rise in recent years since its lowest point in the 2011 to 2013 monitoring period. This change is attributable to both population growth in the region, which has added to traffic congestion, as well as climate change, which has created more ideal temperature conditions for ozone formation.

PM2.5 has also seen significant improvements in the long-term yet worsening conditions in recent years. Overall, the frequency of unhealthy PM2.5 days has fallen 51 percent from 20.8 unhealthy days per year between 2000 and 2002 to 10.2 unhealthy days per year between 2006 and 2008. Levels have jumped significantly, however, since their lowest point during the 2014 to 2016 monitoring period (1.8 unhealthy days) as a result of record-breaking wildfires in 2017, 2018, and 2020.

What is Mountain View doing to improve air quality?

Renewable energy from solar, wind, geothermal, and biomass sources is zero-emission, thus offering significant savings to our environment and public health. Shifting away from the burning of dirty fossil fuels and towards electricity generated by renewables can offer perhaps the greatest impact in improving daily average air quality levels.

In 2015, 19.8 percent of Mountain View’s total emissions came from electricity use.⁶ Since then, Silicon Valley Clean Energy (SVCE) has entered the market, offering 100-percent renewable electricity. In 2017, electricity-based emissions dropped to 10.2 percent as a result of this more diverse energy mix.

Under the Assembly Bill 117 passed in 2002, Mountain View residents are automatically registered into a “GreenStart” program in which 50 percent of energy is from renewable sources provided by the SVCE. Residents additionally have the option to opt for a 100-percent renewable option at an added cost or opt out of the program completely. The auto-enrolled “GreenStart” program is the cheapest of the three options.

In addition to expanding the accessibility of renewable energy, Mountain View has also invested in promoting clean transportation options, such as bike lanes, public transport, and charging stations to promote the transition to electric vehicles.⁷ Currently, transportation represents roughly 60 percent of all pollution emissions in Mountain View, making cleaner transport options an effective means for reducing pollution levels in the city.

+ Article Resources

[1] American Lung Association. (2020). State of the Air – 2020.
[2] U.S. Climate Data. (2020). Mountain View.
[3] Takahashi D. (2020, September 10). Why San Francisco had an apocalyptic orange sky. Venture Beat.
[4] Indoor Air Quality - Scientific Findings Resource Bank. (2020). Wildfires. Berkeley Lab.
[5] Cal Fire. (2020, November 3). Top 20 largest California wildfires.
[6] City of Mountain View. (2020). Renewable energy.
[7] City of Mountain View. (2020). Transportation.