Yosemite sees 'very unhealthy' air quality amid Washburn Fire

A heavy curtain of toxic air hung over California's Yosemite National Park on Sunday as the Washburn Fire continued to burn out of control through parched terrain amid hot, dry weather.

"Visitors to Yosemite should reduce or avoid physical activity outdoors," the national park advised.

The air quality in and around the park reached "very unhealthy" levels and the wildfire smoke spread north along the Sierra Nevada toward the Tahoe Basin and into the foothills.

"The smoke is forecast to continue to move northward overnight with southerly winds," said Robert Baruffaldi, a forecaster with the National Weather Service's Sacramento office.

The smoke isn't expected to reach the San Francisco Bay Area on Sunday as offshore winds are blowing off the Pacific Ocean toward the Central Valley, preventing smoke from reaching the region. The sooty air could begin to funnel into the Bay Area on Monday if winds change.

"The plume of smoke is heading north, but could drift into the Bay Area on Monday," the weather service's Monterey office wrote on Twitter.

The concentration of pollution in the air is measured using the Air Quality Index that operates on a scale from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI value of 50 or below represents good air quality, 51 to 100 is moderate, 101 to 150 is unhealthy for sensitive groups, 151 to 200 is unhealthy, over 201 to 300 is very unhealthy and over 301 signals hazardous conditions.

At 6 p.m., the widely used website PurpleAir measured AQI levels above 200 in a few locations around the park and above 150 in several others. Air Quality levels in Bear Valley and Kirkwood were between 100 to 110 while locations around Lake Tahoe were generally between 50 and 75, with a few spots over 100. The federal AirNow.gov site showed unhealthy levels throughout Yosemite National Park.

PurpleAir's numbers are measured in real-time (averaged over the past 10 minutes). AirNow's figures — which are based on Environmental Protection Agency standards — are calculated using a complex algorithm that "uses longer averages during periods of stable air quality and shorter averages when air quality is changing rapidly." Results are updated hourly but delayed compared to PurpleAir.

PurpleAir sells its proprietary sensors to citizens and then uses the data from the monitors to track particulate pollution on a global scale. People can put them indoors or outdoors at their discretion.

The sensors use a laser particle counter to measure the number of airborne particles and then employ an algorithm to calculate a mass concentration based on the count. They differ from the sensors used by AirNow, which measure particulate matter by drawing air through a filter and then weighing the filter.

The Washburn fire was 1,591 acres as of Sunday afternoon and was burning on the southern end of the park near the Mariposa Grove, threatening its 500 giant sequoias.

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