Part IV – Research Gaps and Future Directions

There is a lack of well-designed epidemiologic studies that quantify the association between wildfire smoke exposure and risk of adverse maternal, fetal and child outcomes. The few studies that have been conducted vary with respect to study design, size, exposure assessment, outcome ascertainment and statistical methods including inclusion of important confounders. This heterogeneity results in difficulty to synthesize and evaluate results. Many studies used satellite-based imaging and PM2.5 measurements, while others used proximity to fire events (using residential or school address), or self-report of the indoor smoke smell, which do not accurately measure individual exposure. Choice of exposure measurement can impact the accuracy of the results (Gan, 2017). More uniform approaches for exposure assessment methods to quantify wildfire-specific air pollution and separate wildfire smoke from existing ambient air pollution to better elucidate the health effects of wildfires, and the impact on the life-course, and these need to be relevant beyond high-income settings. Quantification of smoke waves that incorporate the intensity of wildfire-specific PM2.5, the number of consecutive days in which it is experienced and the cumulative smoke waves across a pregnancy or throughout childhood is needed to examine perinatal and childhood outcomes. Consistency across future studies is encouraged to allow for pooling and better comparison of estimates (Weber et al., 2019).
Future research could benefit from both large population studies across time and space, including prospective cohorts in wildfire-prone areas, and investigations of biological mechanisms. Accurate individual-level data regarding behaviors, time activity, housing characteristics, adoption of risk-mitigation strategies including use of indoor air-filter technologies are also needed to target effective interventions. Improved exposure assessment that accounts for both the short term increases attributable to wildfire in the context of the background air pollution will also provide more accurate estimates of the relationship between wildfire smoke and perinatal and child outcomes. Assessment of interventions to reduce exposure such as use of indoor air-filters, personal masks, or behaviors that limit or reduce exposure specifically in communities impacted by wildfires that include pregnant populations and children are urgently needed. Pragmatic designs that assess real-life situations and are generalizable beyond the immediate study population ready for adoption will have immediate impact on policy. Further quantification of the specific toxic exposures that may differ depending on the material being burned in the fires (i.e., biomass versus structure burning) is needed to better understand the relationships between wildfires and adverse health outcomes.
Studies on wildfire smoke and maternal morbidity are particularly lacking. Exposure to traffic related pollution is presumed to be hazard to pregnant women for developing hypertensive disorders of pregnancy according to a recent monograph by the (US) National Toxicology Program (National Toxicology Program, 2019). This conclusion was based on results primarily of studies of PM2.5 and NO2. Although no studies have been conducted on wildfire smoke and hypertensive disorders in pregnancy or gestational diabetes, it is plausible that an association exists based on ambient air pollution studies (Kim et al., 2021, Zhang et al., 2020, Tang et al., 2020). Future studies are needed to quantify these relationships.
More studies of fetal loss and perinatal mortality are needed to examine wildfire smoke in relation to these outcomes and identify potential underestimates of the association between wildfire smoke and other perinatal outcomes such as preterm birth. Additional studies on birth defects with precise exposure assessment and case ascertainment are needed to conclude their risk with relation to wildfire smoke.
Tracking wildfire smoke exposure and effects in these vulnerable periods (periconception, perinatal, neonatal and childhood) into adulthood will help us quantify the cumulative effects of wildfire smoke across the life course. For example, a recent study found that those born extremely preterm (<28 weeks gestation) or extremely low birth weight (<1000 grams) were at increased risk of adverse respiratory health outcomes following wildfire smoke exposure in early adulthood (Haikerwal et al., 2021).
The distribution of wildfire smoke by socioeconomic status is not as well studied; however, infiltration of smoke indoors may be related to age and quality of housing, posing a critical inequality to reduce personal exposure indoors. We need to consider additional factors that may increase susceptibility to wildfire smoke including issues of environmental justice, housing conditions, and structural racism that impacts socioeconomic inequalities and discrimination that results in higher toxic exposures (Gutschow et al., 2021). The intersectionality of pregnancy and childhood with environmental and structural racism should be highlighted when prioritizing research and policy to protect communities from wildfire smoke. Furthermore, there is a paucity of studies from low and middle-income countries (Jayachandran, 2009, Li et al., 2021b). The usual endpoints such as health service use, or disease exacerbation are mostly irrelevant or immeasurable in those settings, and the other measures that are context specific will be required to develop a strong evidence base of the effect on children and in pregnancy.
The psychological stress caused by wildfire events may affect maternal and subsequent birth and child outcomes. This presents a methodological issue to separate these correlated exposures. Further qualitative and quantitative data assessing the effect of wildfire threat stress may help disentangle the effects from stress via acute wildfire smoke exposure.
We recommend future studies address policy relevant questions related to wildfire smoke exposures and perinatal and childhood health outcomes and that vulnerable populations including pregnant people and children are considered a priority when policies are developed.
In conclusion, numerous studies have found adverse health outcomes associated with wildfire smoke during pregnancy and childhood. This evidence is extrapolated from extensive research on ambient air pollution and adverse pregnancy and child health outcomes. Future research is needed to estimate wildfire smoke exposure more specifically coupled with more health endpoints for maternal, fetal, infant and childhood morbidity.