San Francisco Air Quality Today: 5 Essential Facts About The Current 'Moderate' AQI

As of today, Wednesday, December 17, 2025, the air quality in San Francisco is registering in the "Moderate" category. The current Air Quality Index (AQI) is holding steady at a value of 66, primarily driven by concentrations of Fine Particulate Matter (PM2.5). This status indicates that while the air quality is generally acceptable, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution. Residents are advised to check the localized forecasts, especially those living near high-traffic corridors or industrial zones, as microclimates can lead to significant variations across the city. The current reading is a reflection of the Bay Area’s unique geography and the typical weather patterns of the late autumn and early winter season. While the region is not under a formal Spare the Air Alert at this time, the "Moderate" status serves as a reminder that air quality is a dynamic and ongoing public health concern. Understanding the primary pollutants and the meteorological factors at play is crucial for protecting personal health, especially for sensitive groups like children, the elderly, and those with respiratory conditions.

Current San Francisco AQI: A Breakdown of the "Moderate" Status

The Air Quality Index (AQI) is the primary tool used by the Bay Area Air Quality Management District (BAAQMD) to communicate how clean or polluted the air is and what associated health effects might be a concern. The index runs from 0 to 500, with different ranges corresponding to different levels of health concern. The current AQI of 66 places San Francisco firmly in the "Moderate" range (51-100). This is a significant improvement over the "Unhealthy" or "Hazardous" levels often seen during peak wildfire season, but it is not the "Good" air quality (0-50) that health officials aim for.

The Dominant Pollutant: Fine Particulate Matter (PM2.5)

The main culprit pushing the AQI into the Moderate zone today is Fine Particulate Matter, or PM2.5. * What is PM2.5? These are tiny inhalable particles with a diameter of 2.5 micrometers or less. They are so small—about 30 times smaller than a human hair—that they can penetrate deep into the lungs and even enter the bloodstream. * Sources in SF: In the winter, the primary sources of PM2.5 shift from wildfire smoke to local emissions. These include vehicle exhaust from heavy traffic, industrial activities, and, most notably during the colder months, wood burning from residential fireplaces and stoves.

Spare the Air Status and Advisory

As of the latest BAAQMD update, there is currently no Spare the Air Alert in effect for the Bay Area. Spare the Air Alerts are typically called when the AQI is forecasted to exceed 100 (Unhealthy for Sensitive Groups) due to a buildup of ozone in the summer or PM2.5 in the winter. While the alert is not active, the BAAQMD still encourages residents to voluntarily limit activities that contribute to pollution, such as driving and wood burning.

The Critical Role of the Winter Inversion Layer

San Francisco’s geography and meteorology play a decisive role in its air quality, particularly in December. The city’s air pollution is often a local problem exacerbated by a phenomenon known as the *inversion layer*.

How the Inversion Layer Traps Pollution

A temperature inversion occurs when a layer of warm air settles above a layer of cooler air near the ground. * Normal Conditions: Typically, air near the Earth's surface is warmer and rises, carrying pollutants up and away where they can disperse. * Inversion Conditions: The inversion layer acts like a lid, trapping the cooler, pollutant-filled air close to the ground. This prevents the natural ventilation and mixing that would otherwise clear the air. * Winter Impact: In the winter, especially during cold, clear nights, this inversion layer can be particularly strong and persistent. Local emissions from motor vehicles and residential wood burning accumulate beneath this lid, causing the PM2.5 levels to spike and the AQI to climb into the Moderate or even Unhealthy for Sensitive Groups categories. This trapping effect is why even a relatively small amount of wood smoke from a single fireplace can have a disproportionate impact on neighborhood air quality during a stagnant winter day.

Health Recommendations and Actionable Steps for the Moderate AQI

While a Moderate AQI of 66 is not an immediate public health crisis, it is a signal to take precautions, especially for individuals who are more vulnerable to air pollution. The actions you take today can help keep the air clear tomorrow.

Who Needs to Take Action?

The primary health message for the Moderate category is for Sensitive Groups. This includes: * People with heart or lung diseases (such as asthma, COPD, or congestive heart failure). * Older adults and seniors. * Children and teenagers. * People who are active outdoors (including outdoor workers and exercisers).

Key Health Recommendations

1. Limit Outdoor Strenuous Activity: Sensitive individuals should reduce prolonged or heavy exertion outdoors. Consider moving your workout indoors to a gym or home. 2. Monitor Symptoms: Pay attention to symptoms like coughing, shortness of breath, or chest discomfort. If symptoms worsen, contact a healthcare provider. 3. Keep Windows Closed: Even without a Spare the Air Alert, keeping windows and doors closed can prevent outdoor pollutants from entering your home. 4. Use Air Filtration: Running a HEPA air purifier indoors can significantly reduce PM2.5 concentrations in your living space.

Entities and Resources for Real-Time Monitoring

To maintain topical authority and provide comprehensive information, residents should rely on official and real-time monitoring resources. The air quality can change quickly based on wind patterns and the strength of the inversion layer. * Bay Area Air Quality Management District (BAAQMD): The official regulatory agency for the nine-county Bay Area. They issue all Spare the Air Alerts and provide detailed air quality data and forecasts. * Spare the Air (SpareTheAir.org): The public-facing campaign that provides the daily air quality forecast and status. It is the best source for checking for wood-burning bans. * AirNow.gov: A collaboration between the U.S. Environmental Protection Agency (EPA) and other agencies, offering a national perspective on air quality. * PM2.5 and Ozone: These two pollutants are the most common reasons for poor air quality in the Bay Area. PM2.5 is the winter concern, while Ozone is the primary summer concern. By understanding the current AQI, the factors contributing to it (like the winter inversion layer), and the simple steps to take, San Francisco residents can navigate today's "Moderate" air quality and contribute to a healthier Bay Area environment.