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.