Francesca Dominici and Federica Spoto

Francesca Dominici and Federica Spoto

Francesca Dominici is Clarence James Gamble Professor of Biostatistics, Population, and Data Science at Harvard T.H. Chan School of Public Health and Director of the Harvard Data Science Initiative.

Federica Spoto is a postdoctoral research fellow in biostatistics at the Harvard T.H. Chan School of Public Health.

get the blog in your inbox

Subscribe
Blog Post

How exposure to wildfire smoke impacts human health now, soon, and later

The unprecedented Los Angeles fires threaten the health of millions of residents. Two leading biostatisticians unpack the risks in the short- and long-term.
Jan 21, 2025
By Francesca Dominici and Federica Spoto

Amid the devastation of the Los Angeles fires this month – as tens of thousands of people evacuated their homes, businesses, and communities – the County declared a public health emergency due to all the wildfire smoke and particulate matter in the air. With such fires growing more common, people everywhere want to know: What is all that smoke doing to us?

Scientists rate air quality by measuring the presence of fine airborne particulates, namely PM2.5 –particulate matter measuring 2.5 microns or less in diameter – a mix of solid and aerosol pollutants that can wedge themselves deep in the lungs and enter the bloodstream. For comparison, a single strand of hair is about 30 times wider.

In Los Angeles on January 8, PM2.5 levels spiked to over 480 micrograms per cubic meter. Based on a standard developed by researchers at the University of California, Berkeley, that level of exposure over a 24-hour period is equivalent to smoking about 22 cigarettes.

Let’s first look at the short-term consequences.

Wildfire smoke leads to an uptick in emergency room visits, hospitalizations, and medical consultations for respiratory and cardiovascular conditions. Research shows a heightened risk of heart attacks, strokes, and cardiac arrest, which scientists have linked to an inflammatory response from the inhalation of harmful particles. A 2020 study found that exposure to heavy smoke during wildfires increased the risk of out-of-hospital cardiac arrests by up to 70%. The risk was elevated among men and women, adults 35-64 years old, and communities with lower socioeconomic status. Short-term exposure to wildfire smoke is also associated with a higher risk of COVID-19 infection and death and of dying from all causes.

A 2021 Nature study on short-term wildfire exposure in Southern California found that an increase of 10 microgram/m3 in wildfire PM2.5 caused up to a 10% increase in respiratory hospital admissions, 10 times higher than the corresponding increase in PM2.5 from other sources. In other words, being exposed to PM2.5 is more harmful if it is derived from wildfires than other sources.

Cumulative effects

Scientists worldwide have published extensive research on what happens to people in the days after exposure to wildfire smoke. Now, as wildfires become more frequent, many are asking what happens to people blanketed repeatedly by wildfire smoke.

As we might expect, studies of monthly exposures reveal more cumulative respiratory effects and mental health issues – but they also contain new and startling findings. For instance, in rural counties across the United States, one additional day of wildfire smoke is associated with a 2% increase in suicide deaths.

Now, an emerging body of evidence focuses on the health effects related to long-term wildfire smoke exposure. Research shows a higher risk of death from multiple causes, including cardiovascular, respiratory, and endocrine diseases. It shows a heightened lifetime risk of heart disease, lung disease, and cancer. It shows systemic inflammation, which may lead to severe and chronic health conditions. It shows more adverse impacts on mental health and cognitive abilities, raising serious concerns about the effects of wildfires on brain health over time.

Determining cause

When health professionals look at a disease, they seek an explanation, to isolate the cause of disease from potential alternatives – genetics rather than smoke inhalation, for example. Although the great majority of the epidemiological evidence on the adverse health effects of wildfire smoke accounts for confounding (that is, alternative potential causes that might also accompany the smoke), extensive literature shows that both short- and long-term exposure to fine PM2.5 cause adverse health effects. A recent study that relies on methods for causal inference, for instance, found that an annual increase of 10 micrograms per cubic meter of PM2.5 causes a 6-7% increase in mortality risk.

Who is most vulnerable

Firefighters have more health problems related to their prolonged exposure, research shows, such as a progressive decline in lung function and an increase in asthma. People working outdoors are at heightened exposure risk – including agricultural and forest workers, construction workers, grounds maintenance workers, transportation workers, utilities workers, and athletes. 

However, the risks are higher for less self-selecting groups, too. Studies show more impacts on the elderly and their dementia risk, children, and the socioeconomically disadvantaged – along with people with diabetes or existing heart and lung diseases. Pregnant mothers are more at risk for preterm birth and decreased birth weight, possibly due to stress. Several studies show elevated exposure levels among marginalized groups in the U.S., including the less educated. Women and Black people have higher hospital admissions for respiratory diseases related to prolonged exposure. One study found that populations in wealthier U.S. counties were more likely to remain at home during wildfire events than lower-income populations; in addition, they were more likely to search the Internet for information about air quality and health protection.

As scientists and global citizens, we want you to know the health stakes are high. Wildfires have increased in severity and frequency worldwide, and climate scientists believe this trend will likely persist and worsen. Please stay educated and stay safe as we continue to research how this era of wildfires impacts health.

All perspectives expressed in the Harvard Climate Blog are those of the authors and not of Harvard University or the Salata Institute for Climate and Sustainability. Any errors are the authors’ own. The Harvard Climate Blog is edited by an interdisciplinary team of Harvard faculty.