Exposure to "heavy wildfire smoke" raises a person's risk of catching the coronavirus, research suggests.
Mandatory evacuation orders have been implemented in several northern Californian communities while officials work to control a fire that has already incinerated more than 190,000 acres.
This has corresponded with a surge in coronavirus cases in the "Golden state". According to California's governor, infection numbers rose from 5,577 on 22 July 2021 to 7,984 the following day – a 43% increase in just 24 hours.
Writing in the Journal of Exposure Science and Environmental Epidemiology, scientists from the Desert Research Institute (DRI) have now reported coronavirus cases rose by 17.7% in Reno, Nevada – which neighbours California – during a period of "prolonged smoke" from 16 August to 10 October, 2020.
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This comes after a team of German scientists reported 14% of all coronavirus deaths in the UK as of October 2020 could be linked to air pollution.
"Our results showed a substantial increase in the COVID-19 [the disease caused by the coronavirus] positivity rate in Reno during a time when we were affected by heavy wildfire smoke from California wildfires," said lead author Daniel Kiser.
"This is important to be aware of as we are already confronting heavy wildfire smoke from the Beckwourth Complex Fire, and with COVID-19 cases again rising in Nevada and other parts of the western US."
The Beckwourth Complex Fire describes a network of Californian wildfires that is said to have been started by lightning at the end of June. As of 23 July, the complex had burned 105,000 acres and was 98% contained.
The DRI scientists examined whether wildfires that took place in western US states in 2020 were linked to coronavirus cases in Reno.
From 15 May to 20 October, the scientists measured the amount of fine particulate matter circulating in the air as a result of wildfire smoke.
Particulate matter that is 2.5 micrograms (PM 2.5) – 0.0025mm – or smaller can float unseen in the air. This has been linked to an increased risk of catching any airway virus "via modified immune responses", including inflammation.
PM 2.5 may also "enhance the spread and survival" of aerosols that can contain the coronavirus, according to the DRI scientists.
Levels of PM 2.5 were then compared against confirmed coronavirus cases in a large Reno hospital.
Results suggest every 10 microgram increase in PM 2.5 levels per metre cubed across a week was linked with a 6.3% rise in coronavirus cases.
"This corresponded to an estimated 17.7% increase in the number of cases during the time period most affected by wildfire smoke, from 16 Aug to 10 Oct," wrote the scientists.
The 360,000 people who reside in Reno, northern Nevada, may have been exposed to higher PM 2.5 concentrations for longer periods of time than those living in nearby cities.
The results suggest Reno residents endured 43 days of elevated PM 2.5 over the study's duration, versus 26 days in San Fransisco.
"We had a unique situation here in Reno last year where we were exposed to wildfire smoke more often than many other areas, including the Bay Area," said co-lead author Dr Gai Elhanan.
"We are located in an intermountain valley that restricts the dispersion of pollutants and possibly increases the magnitude of exposure, which makes it even more important for us to understand smoke impacts on human health."
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The results remained the same after the scientists accounted for other factors that may influence coronavirus case data, like the number of tests that were carried out.
"We believe our study greatly strengthens the evidence that wildfire smoke can enhance the spread of [the coronavirus]," said Dr Elhanan.
As well as PM 2.5 itself potentially raising a person's risk, wildfires may "encourage people to stay indoors during bad air events", which could "enhance the spread of [the coronavirus] in indoor public places like restaurants and schools".
The closure of public spaces may also mean "people are forced to take shelter from the smoke in their own homes", allowing the infection to be transmitted between household members.
Reno specifically "was not subject to evacuations that would have prevented exposure to smoke".
"We would love public health officials across the US to be a lot more aware of this because there are things we can do in terms of public preparedness in the community to allow people to escape smoke during wildfire events," added Dr Elhanan.