Suffering Air Quality Starts the Week for Yosemite After Wildfire

As the Washburn Fire continued to burn out of control through parched terrain in the sweltering, dry weather on Sunday, a thick curtain of toxic air hung over California's Yosemite National Park, putting the air quality in the area up to more than 200 AQI.

Yosemite visitors were advised by the national park to limit or forego outdoor physical activity.

Air Quality Index 200: Unhealthy

As wildfire smoke moved north along the Sierra Nevada toward the Tahoe Basin and into the foothills, the air quality in and around the park reached very unhealthy levels.

According to Robert Baruffaldi, a forecaster with the Sacramento office of the National Weather Service, the smoke will continue to move north overnight with southerly winds.

On Sunday, it is not anticipated that the smoke will reach the San Francisco Bay Area due to offshore winds coming from the Pacific Ocean and blowing toward the Central Valley. Depending on how the winds are Monday, the sooty air may start to flow into the Bay Area.

The Monterey office of the weather service tweeted that the smoke plume is moving north but could reach the Bay Area on Monday.

The Air Quality Index, which has a scale from 0 to 500, is used to determine the amount of pollution in the air. The level of air pollution and the resulting health risk increase with increasing AQI values.

The well-known website PurpleAir noted AQI levels above 200 and 150 in a number of locations close to the park at 6 o'clock. API 150 is still described as "unhealthy" although mostly for people with certain sensitivities. Bear Valley and Kirkwood had air quality readings between 100 and 110, whereas most places near Lake Tahoe had readings between 50 and 75, with a few places exceeding 100. Yosemite National Park had unhealthy levels, according to the government's AirNow.gov website.

The metrics for PurpleAir are continuously updated and averaged over the past 10 minutes. AirNow's figures - which are based on The statistics for PurpleAir are calculated in real-time and averaged over the previous 10 minutes. The figures from AirNow, which are based on EPA standards, are computed using a sophisticated algorithm that employs longer averages during periods of stable air quality and shorter averages during periods of rapid change in air quality. Results are delayed when compared to PurpleAir but are updated hourly.

To track particulate pollution on a global scale, PurpleAir sells its proprietary sensors to individuals. Depending on their preference, people can place them indoors or outdoors, SF Gate reports.

The sensors count the number of airborne particles using a laser particle counter and then use an algorithm to determine the mass concentration based on the count. They are distinct from AirNow's sensors, which measure particulate matter by passing air through a filter before weighing the filter.

At the southern end of the park, close to the Mariposa Grove, the 1,591-acre Washburn fire was burning as of Sunday afternoon, endangering its 500 giant sequoia trees.