Durations of Smoke Exposure

Sub-daily, Repeated, and Longer-duration Exposures and Health 
 

Wildfire smoke is more temporally and spatially variable than ambient PM_2.5 concentrations. Additionally, over longer periods, wildfire smoke varies in intensity, frequency and duration, whereas ambient PM_2.5 concentrations remain relatively stable (Casey et al. 2024; Sacks et al. 2025). However, much of the health effects evidence for wildfire smoke focuses on daily (i.e., 24-h average) PM_2.5 exposures instead of sub-daily (i.e., < 24-h average exposures), repeated, or longer-duration exposures within and across years.

The scientific information on daily exposures to ambient PM_2.5 and wildfire smoke provide an extensive database of the health risks of wildfire smoke exposures, but there are uncertainties about the relationship between smoke and health for other exposure durations. To date, fewer epidemiologic studies have examined exposure durations other than daily exposures, with limited information on the potential health implications of sub-daily exposures (i.e., < 24-h average) and; the health implications of longer-duration exposures, including repeated exposures within a year (i.e., over multiple days, weeks or months) and across years. While there is extensive evidence for long-term (i.e., annual) exposures to ambient PM_2.5 and a range of health effects (U.S. EPA, 2019 and 2022), wildfire smoke is a more dynamic exposure that is not constant like ambient PM_2.5. As such, the studies inform the understanding of longer-duration smoke exposure and health should be viewed with caution as many do not account for the dynamic nature of smoke exposure, with some averaging exposures over time, which assumes that people are exposed to smoke every day. While smoke exposure may be associated with the health outcomes examined, and noted below, to date, studies have not accounted for the frequency, intensity, and duration of smoke exposure in examining the health effects attributed to longer-duration exposures. 

The initial studies examining other exposure durations include:

• Sub-daily smoke exposure: increase in ambulance discharges for respiratory- and cardiovascular-related outcomes (Yao et al. 2020) and reductions in cognitive performance (Cleland et al. 2022).
• Persistent or repeated smoke exposures: initial evidence of effects on mental health and well-being (Eisenman and Galway 2022).
• Persistent or repeated smoke exposures within/across specific exposure windows: some evidence that smoke exposure during pregnancy is associated with increased risk of pre-term birth and low birth weight (Heft-Neal et al. 2021; Abdo et al. 2019). Some, but not all, of the studies examining these exposure durations, represent initial studies that employed novel approaches to account for the intensity, frequency, and duration of smoke exposures.
• Cumulative smoke exposure over multiple years: increased risk of premature mortality and cancer (Gao et al. 2023; Gao et al. 2024).
• Average smoke exposure over multiple years: some evidence of increased risk of mortality (Ma et al. 2024) and incident dementia (Zhang et al. 2023).
• Repeated smoke exposure over multiple fire seasons: reductions in lung function in wildland firefighters (Adetona et al. 2016). It is important to note that it is difficult to compare a wildland firefighter’s occupational exposure and resulting health effects to those experienced by the general population. 

Long-term Health Implications of Smoke Exposure
 

Concerns also exist as to whether smoke exposures can lead to the development of health effects in the future. Specifically, there are concerns about whether people are at increased risk of developing chronic health conditions (e.g., heart disease or cancer), and the potential impacts on children. To date, relatively few studies have examined the health effects that may manifest in the future after smoke exposure. However, those studies conducted do provide an initial indication of:

• Reductions in lung function in subsequent years following repeated high smoke exposure over a number of weeks (Orr et al. 2020); 
• Increased risk of influenza infection after wildfire smoke exposure (Landguth et al. 2020); and
• Health effects in children including increased risk of respiratory infection before 1 year of age when exposed during specific developmental periods in utero (Lan et al. 2025), increased risk of respiratory effects when exposed during infancy (Berns and Haertel 2024), and impacts on lung development when exposed during adolescence (Black et al. 2017).

Although the relationship between other durations of wildfire smoke exposure and health have not been extensively examined, these initial studies demonstrate that the health effects from wildfire smoke exposure are not limited to short-term exposures. Table 1 provides examples of the types of health effects that could be experienced in response to wildfire smoke exposure. It is not intended to be a comprehensive list. While uncertainties in the evidence remain, specifically with respect to exposure durations longer than a few days, taking preventative actions to reduce smoke exposure can protect your patients’ health. 
 

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