Unhoused People Are Highly Vulnerable to Wildfire Smoke

People experiencing homelessness are significantly more susceptible to the health impacts of wildfire smoke compared to those with homes, according to a new study from researchers at UC San Diego’s Scripps Institution of Oceanography.

The research, published today in The Lancet Planetary Health and supported by the National Institute on Aging, shows that for every small increase in wildfire smoke pollution, unhoused people face a higher risk of emergency room visits than those with housing. This study is the first to quantify the effects of air pollution from wildfire smoke on the homeless population, a group often overlooked in environmental health research. The findings provide evidence to support the need for adaptation strategies such as the clean air centers being piloted by California.

While policy changes have dramatically reduced air pollution from transportation and industry in California, wildfire smoke is becoming an increasingly pressing public health issue as climate change increases the frequency and severity of wildfires in the American West. Exposure to wildfire smoke, even more so than air pollution from other sources, is associated with a litany of health issues from difficulty breathing to heart attack and stroke. 

On days when smoke fills the air, many cities caution residents to stay indoors and use air filters. But for people experiencing homelessness those recommendations can be difficult or impossible to follow.

Tarik Benmarhnia, an environmental epidemiologist at Scripps who co-authored the study, and his research group carried out a study in 2022 that showed unhoused individuals are particularly susceptible to the dangers of extreme heat — perhaps the most direct way that climate change impacts human health. The current study extends this line of research to wildfire smoke and continues to fill in what Benmarhnia described as a “huge gap in the literature” when it comes to documenting the health impacts of climate change on some of the most vulnerable members of society.

“There were a lot of assumptions that people without housing faced additional health risks from wildfire smoke, but little evidence to back that up,” said Benmarhnia.

To investigate how unhoused populations are affected by air pollution and wildfire smoke in particular, the team assembled data from more than 587,000 emergency room visits for two UC San Diego hospitals from 2012 to 2020. This dataset included detailed information about patients, including their housing status. Though San Diego’s unhoused represent less than 0.3% of the county population, they made up more than 12% of the 587,562 emergency room visits analyzed in the study. 

The team combined those data with air quality measurements that distinguished between wildfire smoke and other sources of the fine particulate air pollution that researchers call PM 2.5. For housed patients, they used the patient's ZIP code to determine air pollution exposure. For unhoused patients, they estimated exposure based on where homeless populations were typically located in San Diego County, according to official annual surveys.

To analyze the relationship between air pollution and emergency room visits, the researchers compared the air pollution levels on the day of each emergency room visit to levels on other days that fell on the same day of the week during the same month and year. Next, the team calculated the change in risks of an emergency room visit associated with increases in different sources of air pollution. 

The study found that emergency room visits associated with “regular” air pollution were about the same for those with and without housing, but when it came to wildfire smoke, unhoused people were at a significantly greater risk of visiting the emergency room compared to the housed population. Among those experiencing homelessness, individuals between 45 and 64 years old and those with a mental health diagnosis had the highest odds of visiting the emergency room during a wildfire smoke event. 

“This shows that we need to design specific adaptation strategies aimed at helping this very vulnerable community,” said Lara Schwarz, who led the study while a PhD student in the UC San Diego - San Diego State University Joint Doctoral  Program in Public Health and is now a postdoctoral researcher at UC Berkeley. “Structurally we should obviously take steps to reduce homelessness, but in the meantime we can do more to protect this vulnerable population from climate hazards like wildfire smoke.”

The findings put data to the assumed vulnerability of homeless populations to wildfire smoke. They also highlight the way in which environmental hazards can compound existing social vulnerabilities and deepen inequality. 

Moving forward, the researchers suggest that cities should prioritize the creation of clean air centers specifically designed to meet the needs of homeless populations during smoke events. 

“This means making sure these spaces are welcoming to unhoused folks and making accommodations for things like belongings and pets,” said Schwarz.

The study authors also call for more research into the most effective strategies for protecting vulnerable groups from environmental hazards, and, of course, long-term solutions to create more affordable housing and address the root causes of homelessness.

“Homelessness is a public health issue,” said Benmarhnia, “especially as climate change increases environmental stresses like wildfire smoke and extreme heat.” 

Rosana Aguilera and Alexander Gershunov of Scripps, Emilie Schwarz, Edward Castillo, Jesse Brennan and Theodore Chan of UC San Diego, as well as Andrew Nguyen of UC San Diego and UC San Francisco co-authored the study with Schwarz and Benmarhnia.