The San Francisco Bay Area is home to the worst air quality in the world

The Bay Area has been inhabited by wildfire smoke for several weeks, but in the early hours of Friday morning. The darkness deteriorated and the air quality in most of the area was seriously unhealthy.

The National Meteorological Administration said that under the condition that the wind direction is the smallest in the weather forecast. It is expected that the concentration of dense smoke will continue for a whole day with little relief.

The concentration of pollution in the air is measured using the Air Quality Index (from 0 to 500).

The higher the AQI value, the greater the level of air pollution, and the greater the concern for health.  AQI value equal to or less than 50 indicates good air quality; more than 150 indicates unhealthy; more than 200 indicates very unhealthy, more than 300 indicates dangerous conditions.

According to data from, at 4 am. The AQI was 250 in San Rafael, 217 in West Oakland, 211 in Berkeley, 200 in San Francisco, and 159 in San Jose.

The widely used website PurpleAir measured an AQI above 300 in the Bay Area. And its global map showed that larger areas have the worst air quality in the world.

PurpleAir’s numbers are measured in real-time (average of the past 10 minutes). AirNow’s data (based on the Environmental Protection Agency’s standards) uses a complex algorithm to calculate that. “Longer averages are used during periods of stable air quality, and shorter averages are used when air quality changes rapidly”‘. The results are updated every hour, but compared with PurpleAir, the results are delayed.

PurpleAir sells its proprietary sensors to citizens and then uses the data in the monitor to track particulate pollution on a global scale.

People can decide whether to place them indoors or outdoors.

The sensor uses a laser particle counter to measure the number of suspended particles in the air. And then uses an algorithm to calculate the mass concentration based on the count. They are different from the sensors used by AirNow. Which measure particulate matter by inhaling air through a filter and then weighing the filter.

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