Impact of Wildfire Smoke on Respiratory Health: Research & Interventions

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Climate change has been called one of the greatest threats to global public health,^1,2 as it impacts not only the physical body but also society as a whole.³ In 2021, more than 200 medical journals issued an unprecedented joint statement that urges world leaders to cut emissions in an effort to avoid catastrophic harm to health. The editorial was published in leading journals, including The Lancet, The New England Journal of Medicine, and the British Medical Journal, among others. One year later, the American Medical Association adopted a new policy declaring climate change a public health crisis.¹

As an expression of climate variability, extreme weather events like wildfires have been increasing in frequency, size, and intensity around the world, exacerbated by drought conditions and heat waves.⁴ Globally, the mortality burden from wildfire smoke is estimated to range between 260,000 and 600,000, with wildfires leading to 339,000 annual premature deaths; however, this is considered an underestimate given increases in extreme wildfire events since the study’s 2012 publication date.⁵ Between 2017-2020, 72% of countries saw an increase in human exposure to wildfires.⁵

Several large wildfires with unprecedented scale and duration have occurred in the past few years:

• In 2018 alone, approximately 55,900 fires burned 8.5 million acres of land throughout the United States. Just four years later, the United States experienced another record of 6,255 fires, which burned a total of 7.5 million acres.⁶
• In Australia, the 2019-2020 bushfires named “black summer” endured for three months and spanned 10 million hectares.⁵
• In 2021, fires were mapped in 22 of the 27 European Union member states, burning 500,566 hectares (the second worst in the EU territory in terms of burnt area); Italy was the most affected country in terms of burnt area, followed by Turkey, Portugal, and Greece.⁷
• In Canada this year, more than 42 million acres of the country was burned due to wildfires. This is comparable to the size of Florida and more acreage than in any of the past 40 years of available data.⁸

Depending upon the type of fuel burned (i.e., forests, grasslands, peat bogs, or human-made structures), a variety of chemicals are emitted from wildfire smoke, including particulate matter (PM; suspended solid and liquid particles) and gases, including carbon dioxide, carbon monoxide, nitrogen oxides, and volatile organic compounds.^9,10 Adverse respiratory health outcomes are most consistently associated with wildfire smoke exposure and particulate matter with a diameter of 2.5 µm or less (PM_2.5).¹¹ What are the short and long-term health impacts of wildfire smoke exposure, and what interventions may mitigate adverse effects, particularly in vulnerable populations like children?

Human Health Effects: A Focus on Respiratory Conditions

A 2023 systematic review of 36 studies by Gao et al suggests that the long-term impacts of nonoccupational wildfire exposure is associated with mortality, morbidity (mainly respiratory diseases), shorter height of children, reduced lung function, poorer general health status, and mental health disorders like PTSD.⁵ Epidemiological studies with human population exposure to wildfires were included without any restrictions on age and sex. Studies included in the systematic review, the majority of which focused on health impacts one or two years after wildfire exposure, predominately originated from developed countries. Regarding respiratory function, the systematic review found the following:⁵

• Five studies assessed the long-term impact of wildfire exposure on lung function, with two studies reporting a decrease in lung function (forced expiratory volume in 1 s(FEV1)/forced vital capacity (FVC) ratio) after wildfire exposure.⁵
• One US study observed a 2.2 L per minute decrease in respiratory peak flow among allergy clinic patients one year post-fire exposure. The lung function was more affected in men than in women, and the general health status was poorer in the older population (>41 years) than in the younger population ( ?41 years).⁵

Another interesting 2023 systematic review by Barros et al focuses on the short-term health impacts of wildfire emissions by geographical area.¹² It suggests that the US has reached higher PM_2.5 concentrations than other countries—almost 19-fold higher than the recommended air quality guidelines. Regarding the meta-analysis of respiratory outcomes, the estimated odds ratio showed an overall increased probability of 3% for hospital admission or emergency department/room visits for all respiratory outcomes (all ages) per 10 µg/m³ rise in PM_2.5 during wildfire events. For the elderly, there was a 7% elevation in asthma-related hospital admission. It is important to note the heterogeneity among these studies; further research is warranted.¹²

While epidemiologic evidence continues to connect exposure to PM_2.5 air pollution with respiratory diseases and mortality, experimental evidence suggests this outcome is linked to oxidative effects on cellular function and inflammation.¹³ The evidence for an association between wildfire smoke and respiratory diseases is clearest for acute effects on asthma. Epidemiologic and experimental evidence suggests that wildfire smoke exposure increases allergic predisposition and upper airway or sinonasal disease; most of this research is focused more generally, however, on fine particulate matter PM_2.5 (which can include traffic, industry, and biomass burning) and is not specific to wildfire smoke. Wildfire smoke is technically considered a specific subtype of PM_2.5. Mechanisms of action, not reflected to date in human data, include the disruption of epithelial integrity and downstream effects on inflammatory or immune pathways.¹³

Population-based evidence for an association between wildfire smoke and respiratory diseases is clearest for acute effects on asthma.^13,14 While different methods were used to estimate exposure, most involved combining data from ground-level, fixed monitoring sites for PM_2.5 with meteorological data and satellite-based imaging or physicochemical data; periods of active wildfire smoke exposure were typically compared with non-exposure periods for the same population. Nearly all studies (many conducted in western North America) showed a statistically significant, exposure dose–related increase in risk for diagnosis or exacerbation of asthma after exposure to wildfire smoke.^13,14

A 2022 population-based observational cohort study in The Lancet of more than two million Canadian adults found that wildfire exposure was associated with slightly increased incidence of lung cancer and brain tumors.¹⁵ Cohort members exposed to a wildfire within 50 km of residential locations in the past 10 years had a 4.9% relatively higher incidence (adjusted hazard ratio (HR) 1.049, 95% CI 1.028-1.071) of lung cancer than unexposed populations and a 10% relatively higher incidence (adjusted HR 1.100, 1.026-1.179) of brain tumors.¹⁵

Children are at an increased risk of respiratory effects from wildfire smoke due to smaller airways, a higher metabolic rate, and continued lung development.¹⁵ A 2021 systematic review of 16 observational studies in urban or mixed urban and rural settings suggests that there may be a significant increase in respiratory emergency department visits and asthma hospitalizations within the first three days of a child’s exposure to wildfire smoke, particularly in those younger than five years old. Eye itchiness, nasal congestion, rhinitis, and sore throat were positively associated with wildfire smoke exposure with a low grade of certainty.¹⁵

A 2020 prospective cohort study on the chronic effects of wildfires on lung function suggests that infants aged <2 years exposed to a six-week coal mine fire had lower lung reactance as measured by forced oscillation technique three years after the fire in association with their level of PM_2.5 exposure during the fire.^10,16 Children with chronic diseases are especially vulnerable to the effects of wildfire exposures, which may affect both airway and systemic inflammation.¹⁸ In children with asthma, PM_2.5 from wildfires may be more likely to trigger an asthmatic exacerbation per unit mass than PM_2.5 from other sources, suggesting a possible enhanced inflammatory effect of the wildfire smoke mixture on asthmatic airways.¹⁰

Interventions: Functional Medicine Considerations

Given the increased risk of adverse effects from wildfire smoke, it is important to consider ways to minimize exposure. As with many pollutants, the toxicant concentration and length of exposure time may indicate the extent of impact and inform any treatment considerations. Patients can follow the Environmental Protection Agency’s AirNow website and Air Quality Index, or similar indices in other countries, for current and predicted ambient air quality effects of wildfires.¹⁹ The US Environmental Protection Agency (EPA) advises individuals to stay indoors with windows and doors closed and limit physical activity to decrease inhaled doses of pollutants on smokey days.¹⁹ Masks and indoor filtration devices may also be used.¹⁹

Following a Southern California wildfire in 2003, children who reported wearing masks, using air conditioning, or restricting outdoor time had fewer symptoms.²⁰ Overall, existing literature suggests that the use of surgical masks and respirators are safe in children and adults, with surgical masks providing a roughly 20% decrease in exposure to wildfire smoke and non-fit tested N95 respirators decreasing exposure by roughly 80% for children and adults.²⁰

Another intervention for managing air quality indoors is the use of filtration devices.²⁰ A study on the Hoopa Valley Indian Reservation in California found that in a real-life setting where multiple interventions were possible (mask wearing, portable air filter use, evacuation to a cleaner area), only the use of a portable high efficiency particulate arresting (HEPA) air filter was associated with decreased symptoms.²⁰ A randomized controlled trial in urban American schools found that classrooms including a set of four portable HEPA filters had significantly lower PM_2.5 and black carbon levels, even though baseline levels before filtration were already low.^20,21 A handout on HEPA purifiers is also available to download for IFM members, within the IFM Toolkit. Public information on indoor air filtration options and efficiency can be found on the EPA’s Indoor Air Filtration Factsheet.

Understanding toxicity, taking practical steps to avoid exposures, and utilizing masks or HEPA filters are essential pieces to improving a patient’s health in the event of wildfire smoke exposure. Other personalized treatments may include lifestyle modifications as well as nutritional support to enhance toxicant elimination, such as IFM’s Detox Food Plan.  Learn more about the health impact of pollutant exposures and treatment strategies at IFM’s Environmental Health Advanced Practice Module (APM).