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Can wildfire smoke spread infectious diseases? Study says it's possible and stresses upon need for investigation

It contains living microbes such as bacteria and fungi, which are aerosolized from burning materials like soils, detritus, and wild woods and transported in smoke plumes
(J Juchtzer)
(J Juchtzer)

Exposure to wildfire smoke and other environmental hazards is known to pose a significant health risk to fire personnel. Scientists now warn that wildfire smoke can carry microbes like bacteria and fungi that cause infectious diseases. The team from the University of Idaho and University of California (UC), Davis, explained that while the microbial concentration in smoke is higher near the fire source, these microbes may be active agents spreading infection. 

Smoke from the growing number of annual wildfires across the western US and Australia has led to lengthy periods of unhealthy and hazardous air quality for millions of people living in these regions. It is well-documented that exposure to wildfire smoke can damage the heart and lungs. Respiratory allergic and inflammatory diseases, including asthma and bronchitis, are also worsened by exposure to smoke. But in the perspective, published in Science, researchers have called the attention of the scientific community to the little-known and poorly understood threat potentially lurking in the plumes: infectious microbes. They argue that while the pulmonary and cardiovascular consequences of smoke exposure are well known and recognized, the potential for wildfire smoke to be a source of infection has been overlooked and remains unaddressed in public health and wildfire science.

According to the experts, very little research has been done to date for determining whether the transport of smoke-borne microbes poses a health risk, in addition to the risk known from particulate inhalation, despite “compelling evidence” that shows increasing rates of certain fungal infections in areas with increased levels of wildfire smoke. 

The investigators note that the potential for a wildfire’s microbial content to affect humans who breathe in smoke, particularly from large fires and over long periods, is ‘appreciable.’ They say given that climate change impacts on wildfire are predicted to lead to total emissions (greenhouse and trace gases plus particulate matter) increases of 19 to 101% in California alone through 2100, it is important that atmospheric and public health sciences expand their perspectives to include the potential impact of smoke’s “microbial cargo” on human populations. This is especially relevant where smoky skies are more likely to be a seasonal norm rather than a rare event, add experts. 

Diverse microbial colonies grown from smoke samples taken during wildland fire (LN Kobziar)

“With longer wildfire seasons and higher severity trends, there is an urgency to work together in studying the behavior of the microbes carried by the smoke and their impact on human health,” suggests George Thompson, associate professor of clinical medicine at UC Davis.

Role of microbes in wildfire smoke

According to the authors, wildfire smoke contains living microbes that are aerosolized from burning materials such as soils, detritus, and wild woods and transported in smoke plumes. “Wildland fire is a source for bioaerosols that differ in composition and concentration from those found under background conditions, and most of these microbes in smoke are viable. Bioaerosols, composed of fungal and bacterial cells and their metabolic by-products, are known to affect human health,” they write.

Once suspended in the air, particles smaller than 5 μm can travel hundreds or even thousands of miles. Their movement depends on fire behavior and atmospheric conditions. Eventually, they are deposited or inhaled. Bacteria and fungi can be transported in these wildland fire smoke emissions. The US Centers for Disease Control and Prevention (CDC), for example, counts firefighting as an at-risk profession for coccidioidomycosis. A fungus that becomes airborne when soils are disturbed, coccidioidomycoses is the cause of Valley fever, a potentially serious infection.

“We don’t know how far and which microbes are carried in smoke. Some microbes in the soil appear to be tolerant of, and even thrive under, high temperatures following wildfires,” emphasizes Thompson.

Leda Kobziar, associate professor of Wildland Fire Science at the University of Idaho, further explains, “At the scale of a microbe, fire behavior research has shown that heat flux is highly variable, so it may be that many microbes aren’t even subjected to the high temperatures for very long. They may also be protected in small clusters of particulate matter.”

View of flaming and smoldering combustion which aerosolize living microbes from soils and vegetation (DRI ASTER Lab)

The authors suggest that addressing these unknowns will require a multidisciplinary approach representing expertise in fire ecology, environmental microbiology, epidemiology, public health and infectious disease, and atmospheric sciences. “The knowledge gained has the potential to answer questions about the consequences of wildland fire specific to each of these disciplines. For example, what roles does fire play in the spread of disease, and can natural reservoirs and affected populations be linked through smoke to predict public health problems before they occur? Exploration of infections and indicators such as antibiotic use in populations subjected to known amounts and durations of wildfire smoke is a promising first direction,” note scientists.

What are other experts saying?

According to Dr Joel Rindelaub, an aerosol chemist at the University of Auckland, the report ‘reinforces’ the hazards of breathing in smoke from forest fires. “Finding living fungi and bacteria in wildfire smoke isn’t completely unsurprising, as we already know that they are present in the atmosphere. For instance, these types of particles make up 20% of the particulate matter in the sky, and up to 75% of the particles above forests. What’s more surprising is that not only do these tiny living creatures survive in smoke, but the smoke itself can act like an airborne fishing net that collects microbes and transports them hundreds of kilometers,” emphasizes Dr Rindelaub. He warns that as climate change continues, one should expect more bushfires and more exposure to “these types of harmful species.”

Dr Richard Winkworth, senior lecturer in plant genetics at Massey University, states that while the study focuses on the human health aspects of pathogens being spread as the result of wildfires, there is potentially a much wider range of impacts. “If human pathogens are capable of being aerosolized and carried hundreds of kilometers, then those affecting our native biota or primary production could also be spread this way,” adds Dr Winkworth. 

Dr Tara Strand, general manager of forests and landscapes at Scion, a New Zealand-based research institute, says that the “demonstrated link” between emissions of viable microbes and wildland fire and their transport within smoke plumes is of “concern for biosecurity reasons.” “One of the primary tools in a biosecurity response to a pathogen attack is to remove and burn. To date, the majority of these burns have been in the open atmosphere. This begs the question of, when doing a remove-and-burn biosecurity response, what is the sensitivity of these pathogens to heat, and do they remain viable, and what is their inoculum load, in the smoke plume?” emphasizes Dr Strand.