Qualité de l’air à Napa County

How is the air quality in Napa?

Napa, the seat of Napa County, is located in Northern California, approximately 48 miles north of San Francisco. Napa County is part of a wider region known as Wine Country, named after its globally recognized wine exports.

Napa features a generally warm and temperate Mediterranean climate, with winters tending to be rainier than the summer months.^1,2

Air pollution in Napa County receives mixed assessments. The American Lung Association (ALA) rated the county a “B” for ozone and an “F” for 24-hour particle pollution.³ The ALA assigns pollution grades by calculating a rolling three-year average of data for multiple types of pollution, then ranking air quality in the USA by compiling statistics for every county in the nation.

To achieve an “A” rating, the county must have zero unhealthy air quality days. Historically, ground-level ozone formation has not been as prevalent in Napa County compared to other parts of the Bay Area closer to San Francisco, Oakland, and San Jose.

The data behind Napa County’s “B” grade for ground-level ozone (smog) showed that, in three years, the county averaged just two days of unhealthy air for sensitive populations, and no days with stronger health warnings.

Napa County nonetheless experiences consistent fine particulate matter (PM2.5) pollution.

From 2016 to 2018, the county averaged 10 days of air that was unhealthy for sensitive populations, 13 days when air was unhealthy for all, and two days when air was designated as “very unhealthy.”

An increase in wildfire smoke during this period, caused by events such as the October 2017 Tubbs, Partrick, Nuns, and Atlas fires, was a major contributor to Napa County’s failing grade, along with woodburning and other sources.⁴

To know what air quality in Napa Valley is, check this page regularly for data describing how healthy the air in Napa is at the present time and the Napa air quality index (AQI) for today as well as for the rest of this week. When pollution levels are high, follow recommend health advice to reduce your pollution exposure.

Is today a Spare the Air day in Napa?

Spare the Air days occur when PM2.5 particle concentrations rise to unhealthy levels. This can happen in Napa County due to several interdependent factors:

• Wildfires often cause a rapid rise in air pollution, which occurred in late summer 2020 when the Hennessey, Walbridge, and other wildfires triggered a Spare the Air alert.⁵
• Winter months bring increased woodburning and winds that can cause air saturated with fine particle pollutants to move from the Central Valley to southern Napa County.
• Since most of Napa County (including Napa itself) is too far north of the San Pablo Bay (located on the county’s southern border) to enjoy its moderating effect on temperatures, residents endure some of the coldest nights in the Bay Area. This leads to increased woodburning in fireplaces to generate heat. The resulting smoke is a major cause of elevated PM2.5 concentrations in the air.⁶

If one or more of these factors causes PM2.5 to rise, and these tiny particles are forecast to reach unhealthy levels, the Bay Area Air Quality Management District (BAAQMD) will announce a Spare the Air alert. When such an alert is issued, the BAAQMD warns the public about this dangerous rise in air pollution and bans woodburning, one of the Bay Area’s two biggest sources of fine particulate pollution along with wildfires.⁷

In 2019, the BAAQMD adopted new amendments to strengthen its Wood Burning Rule in an effort to cut down on wood smoke and thus reduce the number of Spare the Air days. This is especially important during wildfires in order to improve the air quality in Napa and other Bay Area cities and protect public health.⁸ Under the toughened rule, burning wood, manufactured firelogs, pellets, or solid fuel is illegal when a Spare the Air Alert―Burn Ban is in effect. Those who ignore the rule are subject to stiff penalties.

When a Spare the Air alert is in place within the Napa area, people should reduce or forego outdoor activities as much as possible to protect their health, and limit pollution emitting activities which may contribute to the city’s poor air quality.

Is there smoke in Napa Valley?

There are two primary causes of smoke in Napa Valley and other parts of the Bay Area:

• natural wildfires: Common during summer when daylight hours are long and temperatures soar
• household woodburning: Common during winter when cold temperatures increase demand for fireplace heat

Woodburning is relatively controlled, while wildfires are unpredictable and can quickly become uncontrolled if not contained quickly.

When wildfires burn in Napa Valley or nearby areas of Northern California, several factors determine how quickly the smoke from those fires will clear. Napa Valley’s weather patterns and geography are two of the biggest determinants.

Winds are generally calm throughout the county, reducing the flow of fresh air conducive to clearing out smoke. In addition, much of Napa County is sheltered from winds due to the mountains surrounding Napa Valley, and this can trap PM2.5 particles within the region.

When summer brings more heat, drier conditions, and less frequent rainfall, more wildfires occur, and smoke tends to be present in the air more often.

In contrast, during colder months, wildfires are less frequent, and since Napa winters are rainier on average than summers, smoke tends to clear away faster due to the beneficial effect of rain on smoke particles. This effect, known as precipitation scavenging, helps reduce the ratio of PM2.5 particles (small-sized pollutant particles that can cause long-term damage to the body) to PM10 particles (larger-sized pollutants that generally cause short-term irritant effects).⁹

If the BAAQMD has issued a Spare the Air alert for PM2.5 pollution, there is a high probability of smoke in the air. Remain indoors as much as possible and avoid strenuous outdoor activities in order to reduce the risk of breathing unhealthy airborne pollutants.

What is causing air pollution in Napa?

Unlike some regions in the southern Bay Area, Napa County only rarely sees ozone rise to unhealthy levels. When this does occur, it’s usually in the summertime when sunlight and heat can cause nitrogen oxides (NO_x) and volatile organic compounds (VOCs) to react and form ozone.

Since there is less vehicle traffic in Napa Valley than in much of the Bay Area, NO_x and VOCs are present in much lower concentrations, which generally leads to less ozone formation. Similarly, since these compounds only form ozone when exposed to sufficient levels of sunlight and heat, less ozone tends to form during winter months.

In Napa County, unhealthy levels of PM2.5 can and do occur at any time of year due to multiple factors that can cause regular increases in PM2.5 concentrations.

Wildfires often cause a rapid rise in PM2.5 levels. Wildfires tend to occur more often in the summer and early fall when temperatures rise in the Napa region and surrounding areas, leading to drier conditions that make fires easier to start and spread.

Since most of Napa County is not in close proximity to a body of water that can help keep temperatures in a moderate range, the county tends to have colder nights than more southerly parts of the Bay Area near the San Francisco and San Pablo Bays, especially during winter months. This leads to an increased likelihood of woodburning in fireplaces, a major factor in increasing PM2.5 particulate matter to unhealthy levels.

Wildfires and woodburning are the two biggest sources of air pollution in Napa, as they can cause immediate spikes in PM2.5

In the winter months, winds also tend to become stronger and more frequent. Such winds can bring air saturated with fine particle pollutants from the Central Valley to southern Napa County.

Is air quality in Napa getting worse?

Napa County’s relatively good rating for ground-level ozone shows that smog is not as prevalent in this region as in other parts of the Bay Area, and this has been the case for many years.

The annual weighted average number of High Ozone Days decreased from 3.7 in 1997-1999 to 0.7 in 2000-2002 and has stayed at or near that 0.7 average for nearly every three-year period since then.

Napa County has fared less well for 24-hour fine airborne particulate matter, receiving a failing grade for this type of pollution due primarily to increased wildfires.

However, the trend of increased smoke from wildfires has occurred alongside reduced emissions from woodburning during winter months. Napa County particle pollution could experience marked improvement if the frequency of major wildfires is reduced.

Ground-level ozone in Napa County has held relatively steady for decades and continues to not be a major problem, while elevated levels of PM2.5 particulate matter has become a health concern in the region. But improvements in PM2.5 are possible as authorities crack down on woodburning, the number of electric cars on California roads increases, harmful emissions are reduced, and remote work becomes more common, resulting in decreased traffic and an even greater reduction in vehicle emissions.

+ Article Resources

[1] City of Napa. (2020). About Napa.
[2] Climate-Data.org. (2020). Napa climate: Average temperature, weather by month, Napa weather averages.
[3] American Lung Association. (2020). State of the Air report.
[4] Eberling B. (2020, July 31). Napa County gets mixed grades in air pollution report due to wildfire smoke. Napa Valley Register.
[5] Yune H. (2020, September 28). Update: Wildfire smoke creeps into daily Napa life. Napa Valley Register.
[6] Bay Area Air Quality Management District. (2019, February 14). Napa County.
[7] Bay Area Air Quality Management District. (2020). An overview of the Wood Burning Rule in the Bay Area.
[8] Bay Area Air Quality Management District. (2019, November 20). Air District strengthens wood burning rule to protect air quality and public health during wildfires.
[9] Becerra LCB, et al. (2015, December). Influence of precipitation scavenging on the PM2.5/PM10 ratio at the Kennedy locality of Bogotá, Colombia. DOI: 10.17533/udea.redin.n76a07.