It's no secret that inhaling smoke is bad for your lungs. But now, scientists are suggesting smoke may also carry and spread infectious diseases.
The theory, published in Science Magazine, is based on research that found wildfire smoke is teeming with thousands of species of microorganisms. Some of these microorganisms, including bacteria and fungal spores, are known to cause disease.
Until now, it's been widely accepted that the greatest risk to human health from smoke are the tiny particles from burning wood and other materials. Some of those particles are small enough to be inhaled into the lungs, triggering allergic reactions in some people or exacerbating lung diseases such as asthma in others.
The new research posits that when a wildfire burns plant or animal matter and disturbs soils, it exposes thousands of species of bacteria and fungi that otherwise might not easily become airborne. You might think the high heat from fire would kill these organisms, but one study cited in the article found that some bacteria even multiply post-fire. Scientists say the organisms latch onto smoke particulates, allowing them to travel thousands of miles across continents.
The possibility that smoke-transported microbes may be contributing to infections in human populations has yet to be studied. But it should be, the authors of the article argue. In some locations, growing rates of fungal diseases have coincided with increasing wildfire smoke — what the authors of the article call "compelling overlaps." The U.S. Centers for Disease Control and Prevention also warns that firefighters are at higher risk of contracting valley fever, an infection caused by inhaling a fungus found in soil.
"I think that the connections haven't been made in the past because it's a very new idea to think of smoke as having a living component," says Leda Kobziar, co-author of the article and associate professor of wildland fire science at the University of Idaho.
Kobziar hopes that doctors and scientists will devote more research to whether prolonged smoke exposure can cause infections and whether the spread of disease can be predicted from smoke patterns.
Dr. Peter Chen, director of the Division of Pulmonary and Critical Care Medicine at Cedars-Sinai Medical Center in Los Angeles, is "intrigued" by the theory but somewhat skeptical that the microorganisms in smoke would actually cause infections. Many bacteria and fungi don't cause lung infections, says Chen, but it's certainly possible that a significant dose could exacerbate symptoms in someone with a preexisting lung condition.
"I always thought it was the particulates in smoke that were causing these issues," says Chen, "but when I read this, I started thinking could it be the microorganisms that are also causing exacerbations?"
Whether the microbes in smoke actually cause infection or simply aggravate underlying respiratory issues, the article raises a new health threat that is "certainly alarming," says Kelsey Jack, an associate professor of environmental and development economics at the University of California, Santa Barbara and co-chair of the climate group at the Abdul Latif Jameel Poverty Action Lab.
This is especially true for lower-income populations, Jack says, because people with fewer means are often more exposed to the environment. If smoke is affecting the air quality in a certain area, the people who work outside, or who have to commute by foot or bike, will inhale more smoke than those who commute by car to their office jobs.
Additionally, air pollution tends to be worse in developing countries — meaning those populations have a lower baseline for respiratory health. Pneumonia is the No. 1 cause of death in children under 5 years old, and respiratory disease and infections are a leading cause of death among aging adults in poorer nations.
Jack says it would also be helpful to study whether smoke from other common sources in developing countries — such as indoor cooking fires or stubble burning, when farmers set their fields on fire after harvest season to clear them — contain as many infectious microbes as wildfires. And if so, whether it's contributing to disease in those populations.
"To the extent that this is a missing piece of understanding the relationship between an emission source and a health impact, it is very, very valuable," says Jack.
But until more research is done, Chen says the best thing people can do is just follow existing recommendations when air quality is poor — including staying indoors, keeping windows and doors closed, using HEPA filters and running air conditioning.
"Really, we just want people to scrub the air," says Chen.
According to the article, further study is especially relevant as climate change will likely make smoky skies a "seasonal norm rather than a rare event" in places around the world.
"Given the fact that we will be living with smoke, we would just encourage people to take precautions," says Kobziar.