Part III – Exposure to wildfire smoke and child health
outcomes
The epidemiological evidence from studies investigating wildfire smoke
exposure on child health is heterogeneous and limited. Most research has
been conducted in high-income countries in Australia, Europe, and North
America and assesses, most commonly, acute outcomes close to the time of
exposure (hospital presentations or admissions, or respiratory
symptoms), and uses a mix of methods to assess outcome and exposure
assessment that make estimation of summary effects difficult. There have
been some studies conducted to examine longer-term outcomes however
these mostly focus on wildfire as a traumatic event and the impact on
mental health outcomes and are not necessarily wildfire smoke exposure
investigations. As a body of evidence there are many gaps in our
understanding of the impact of wildfire smoke exposure and child health.
However, most literature acknowledges that childhood is a recognized
window of susceptibility, and that children have particular
vulnerabilities to wildfire smoke due to their immature immune system,
developmental stage and their behaviour (such as outside play), and that
wildfire events are increasing and therefore more children will be
impacted (Liu et al., 2016, Holm et al., 2021).
Vulnerability during
childhood
Early childhood is a vulnerable period of growth and development, and
children are particularly sensitive to environmental exposures
(Sheffield and Landrigan, 2011). Young children’s smaller airways,
smaller lung size, higher respiratory rates, increased metabolic rates,
and their under-developed immune system compared with adults increases
their exposure and vulnerability to respirable pollution (Leibel et al.,
2020). Young children are also less likely to adopt risk mitigation
behaviors, have less access to appropriately sized masks, and may spend
a large proportion of the time during an exposure period where the air
is insufficiently ventilated (in school), or outdoors (Sheffield and
Landrigan, 2011, Holm et al., 2021) and for these reasons are at greater
risk of exposure, and more vulnerable to the risks.
Particulate matter, one of the most frequently measured solid particles
found in air, are common in both ambient air and wildfire smoke. Levels
of particulate matter and adverse health outcomes are linearly related
and the risk for adverse health increases as concentrations of
particulate matter increase regardless of the source (Gan et al., 2017).
Although the evidence from wildfire smoke is increasing (Reid et al.,
2016a), much of the earlier epidemiological evidence, comes from studies
of ambient air pollution (Stieb et al., 2012). Although these studies
are useful for hypothesis development of the effect of wildfire smoke
exposure there are some important differences between these exposures.
For example, the chemical composition of wildfire smoke is similar but
not the same as other causes of air pollution including traffic,
agricultural burn-off, and household air pollution (Künzli et al., 2006)
and within these, the composition may vary based on type of biomass
burned, whether structures were also burned and the duration of the
burning event. Wildfire smoke is more likely to have a shorter duration,
whereas exposure to traffic pollution is more constant. Examining
short-duration, unpredictable exposure on acute health outcomes is also
challenging, as is examining the longer-term impacts into adolescence
and adulthood. The impact on adult health after short-term exposure in
childhood is also unknown (Shao et al., 2019). Animal models of early
life exposure and longer-term morbidity are suggestive of immune
dysregulation and changes to lung function that extend at least into
adolescence (Black et al., 2017a). Currently, most of the
epidemiological research investigating longer-term effects has
investigated mental health, or psychological outcomes due to exposure to
trauma (of wildfire threat), rather than other morbidity, although all
health outcomes are important areas for research.
Epidemiological evidence of childhood health effects of
wildfire smoke
Health Service Use
Health service use is a frequently assessed outcome for epidemiological
studies of wildfire exposure on children. Health service use is defined
mostly through hospital, or emergency department presentations or
admissions, and or doctor or clinic visits in children during or related
to a wildfire event or smoke exposure. These episodes of care are mostly
related to respiratory causes, and health service use in general and
specifically related to respiratory symptoms has been reported to
increase during periods of wildfire smoke exposure and comes from a
range of studies (Henry et al., 2021). A prospective cohort study of
>6000 school-aged children, mostly aged 6 to 7 years,
collected annual respiratory health data and included wildfire exposure
following the 2003 Californian wildfires (Künzli et al., 2006). All
respiratory symptoms and doctor visits were increased in children
affected by smoke by up to 33% (Künzli et al., 2006). There were
differences by asthma diagnosis and children’s age (e.g. high
school children reported more eye symptoms than primary children, 44.0%
versus 35.5%). Children with asthma reported more respiratory symptoms,
but this was modified by adoption of preventative behaviours (mask
wearing, staying indoors, and using air conditioners), and the number of
reported days of fire smell indoors. In children who were the most
exposed (>6 days of smoke, and without masks), sneezing
more than doubled [OR=2.8 (95% CI: 2.3, 3.5)]. The results from
this study are generalizable to a range of similar settings as it was
population-based study. A recent ecological study also from California,
reported increases in pediatric (<19 years old) emergency
hospital presentations by 30% when the PM2.5attributable to wildfires increased by 10 ug/m3(Aguilera et al., 2021). During the exposure period, younger children,
those between 0 to 5 years were the most likely to present to hospital
(34.5% increase) (Aguilera et al., 2021). Respiratory presentations of
cough increased 21.8% with a 10 ug/m3 increase in
PM2.5 attributable to wildfire smoke (Aguilera et al.,
2021). A study using medical insurance claims for episodes of care
related to respiratory or cardiac outcomes for people aged 0 to 64 years
during a six-month window that included the wildfire season in San Diego
reported that children’s attendance at outpatient clinics for
respiratory conditions was greater during peak fire-exposure periods.
Overall children aged 0 to 4 years accounted for 14.4% of all inpatient
hospitalizations, 15.1% of emergency presentations, and 28.8% of
outpatient visits. Children younger than 2 years had higher risk of
presenting, although the numbers of children in this age group were very
small and varied by the specific respiratory condition for presentation
(Hutchinson et al., 2018). A larger interrupted time series study also
from San Diego, used hospital data for all children (<19 years
old) to quantify excess daily hospital visits for respiratory symptoms
during a 10-day fire exposure period. Daily emergency department visits
increased by 16.03 daily visits per 10,000 children, during the exposure
window. Younger children aged 0 to 5 years had the highest absolute
daily visits of 7.3 daily visits. Children aged 6 to 12 years had the
most change in visits, which changed by 3.4 (95% CI: 2.3, 4.6) daily
visits (Leibel et al., 2020). In a study to assess different simulation
models using hospital respiratory admissions in Washington the odds of
respiratory hospitalizations in children (<15 years) increased
by ~10% on days when the PM2.5increased by 10 ug/m3, which was found consistently in
all three simulation models (Gan et al., 2017). In contrast, an older
population-based study used hospital admission and mortality data over
eight years, from Sydney, Australia that included 32 bushfire days and
found bushfire derived PM10 was associated with moderate
decreases in asthma hospital presentations for children three days after
the peak event (Morgan et al., 2010).
In epidemiological studies of the association between wildfire smoke and
health, PM2.5 is the most common measure of wildfire
smoke exposure; however, increases in ozone also occur during wildfires
(Pratt et al., 2019). Ozone is a secondary pollutant and fluctuates with
temperature, time of day and weather conditions, and wildfires are an
additional source of ozone (Pratt et al., 2019). A 2019 study used
analytical models to estimate the burden of pediatric emergency
department visits attributable to elevated ozone associated with
wildfire smoke in the US, between 2005-2014. They found associations
between ozone that was attributable to wildfire smoke and increases in
emergency department visits of children <18 years presenting
for asthma (Pratt et al., 2019). The overall median number of visits for
the ten years of the study period was 29 emergency department visits per
child [(interquartile range (IQR): 3-63)]. The number was higher for
males, 18 visits (IQR: 2-40) than females, 10 (IQR: 1-20) visits (Pratt
et al., 2019).
Respiratory Symptoms and Asthma Medication
Use
Morbidity based on respiratory symptoms, exacerbation of existing
disease, new disease diagnoses, and medicine use associated with
wildfire exposure in children has mostly been examined through
self-report of these outcomes. A 2016 Spanish birth cohort study of 496
children, whose parents completed a telephone interview two weeks after
a wildfire event, reported an increase of respiratory symptoms for all
children. Children with existing asthma were nine times more likely to
report itchy or watery eyes [OR=9.26 (95% CI: 2.14, 40.12)], or 11
times more likely to report sneezing compared to children without asthma
associated with smoke from wildfires [OR=11.40 (95% CI:
2.01-64.52)] (Vicedo-Cabrera et al., 2016). A 2009 longitudinal study
in California of 465 non-asthmatic children aged 16 to 19 years who
self-reported respiratory and eye-symptoms following a wildfire event,
demonstrated a link between duration of smoke exposure and respiratory
symptoms (Mirabelli et al., 2009). For children who reported smelling
smoke for six or more days (the exposure variable) the prevalence of
respiratory symptoms was 60%, for eye symptoms the prevalence was 80%
(Mirabelli et al., 2009). The Californian study by Künzli et al.described above reported that respiratory morbidity including dry cough,
wheeze and eye-symptoms were more common in children exposed to wildfire
smoke than those not exposed and was higher in children with existing
asthma (Künzli et al., 2006). A panel study of 235 people in Darwin,
Australia of whom ~50% were <18 years old,
asthma medication use both reliever medication and initiation of oral
steroids were associated with low levels of PM10 and
PM2.5 derived from wildfire smoke (Johnston FH et al.,
2006). A 2015 retrospective study of two cohorts of >6000
Californian children where exposure to air pollution was determined by
ZIP codes and distance to smoke plumes one year before and one year
after wildfires analyzed several asthma outcome measures. These included
short-acting β-agonists dispensing behavior. Dispensing behavior
following wildfire exposure increased in all asthmatic children but the
result was inconsistent between the two cohorts (Tse et al., 2015). In
an older and very small study (n=32 children), asthmatics exposed to
excessive PM10 from bushfire smoke for seven days did
not find any association with peak
expiratory flow rates; however, among a subgroup without bronchial
hyperreactivity, there was an inverse association between
PM10 levels and evening peak expiratory flow rates
(Jalaludin et al., 2004). In more recent work, 1404 children aged 4 to
21 years completed a Forced Expiratory Volume in 1 second test (FEV1) in
a three-day window after wildfire smoke exposure. Children aged 12 to 21
years had poorer lung function on day one which returned to normal
function by day three. Children 4 to 11 years did not demonstrate any
change in lung function (Lipner et al., 2019).
There is limited data of the effect of wildfire smoke exposure on child
mortality. A recent study estimated the under-5 child mortality
attributable to wildfire and managed landscape burn smoke exposure, and
reported an estimate of 104,279 child deaths a year, which accounts for
39% of the global annual mortality burden attributable to wildfire and
landscape burns smoke (677, 745 annual premature deaths) (Roberts and
Wooster, 2021). This finding comes from a 2021 modelling study that used
statistical methods to isolate the contribution of managed landscape
burning to overall PM2.5 and then calculated the number
of people exposed to estimate mortality by country. Most of these deaths
occur in low- and middle-income countries, and the data suggest that
landscape biomass burning is responsible for between 8-21% to the
population weighted PM2.5 concentration in many
countries, making it the biggest contributor to all ambient air
pollution. More accurate data and further testing of the modelling will
increase the accuracy and precision of these estimates.
Other vulnerabilities and indirect
outcomes
Some studies have investigated potentially vulnerable population
sub-groups, including in obese children (Tse, 2015). Most studies
investigating body mass index (BMI) as a risk factor in children have
examined other air pollutant exposures; such as tobacco smoke or
PM2.5, and not specifically wildfire smoke exposure
(Huang et al., 2019). Obesity is as a risk factor for asthmatic disease,
both incident disease and severity (Sin and Sutherland, 2008), and the
prevalence of childhood obesity is also increasing (WHO, 2018) which
suggests that there is an urgent need to conduct these investigations to
better understand risk factors.
In two retrospective cohorts of children aged 5 to 17 with asthma in
Southern California included 1050 obese children,
as described above (Tse et al.,
2015). Overall, dispensing of short-acting ß-agonists was increased in
the post-fire periods, and the greatest increase in dispensing was in
children who were obese (16% increase, P<0.05); however, when
this was restricted to children only in the fire ZIP codes, there was no
difference in dispensing by BMI. Overall, there was no increase in
hospital presentations or admissions, oral corticosteroid dispensing
frequency, or new asthma diagnoses post-wildfire (Tse et al., 2015).
Impacts from wildfire or prescribed burning on the immune system and
cardiovascular health were investigated in a small convenience sample of
220 children (mean age 7 years) in California, reported as a letter to
the editor (Prunicki et al., 2019). Of the 32 children who had been
exposed to wildfire pollution or prescribed burning (based on residence)
three months prior compared to 18 children with no exposure demonstrated
a difference in Th1 cells, but not in Th2 or T regulatory cells
(Prunicki et al., 2019). This was a small study suggest potential immune
changes, though the lack of details make the study difficult to evaluate
and results need to be further investigated.
The effect on mental health from exposure to wildfire evidence has been
assessed mostly through the lens of experiencing a traumatic event.
Trauma related to wildfire exposure has been defined using the full
range of experience, from living in the fire or smoke affected
community, being evacuated, perception of threat, through to the death
of a parent. Estimates of post-traumatic stress disorder, the most
studied mental health outcome in this field, is estimated to be between
12-30%, although this varies considerably with the measures used, the
age of the children, and definition of the trauma experience. In one
cross-sectional study conducted in Greece, 1339 adolescents aged 12 to
17 years completed a self-report questionnaire of wildfire exposure and
symptoms of threat, or perceived threat including anxiety and
depression, six months after a wildfire event. Of those 432 adolescents
(29.4%) reported levels above the cut-off of post-traumatic stress
disorder symptoms, and perception of life threat was a greater predictor
of these symptoms than actual threat (Papadatou et al., 2012).
Academic achievement was assessed four years after exposure to a
bushfire disaster in a recent Victorian, Australian population-based
study of 24,642 children who were highly or moderately exposed to a
major bushfire. Exposure was assessed through school proximity to the
bushfire. Academic achievement was measured using the standardized
annual literacy and numeracy test scores at two time points two years
apart, two and four years after the bushfire for each child. There were
reductions in the children’s expected numeracy and literacy test scores
based on the expected trajectory between the two test scores at
approximately ages 9 and 13 years. The hypothesized impact pathway was
through cognitive deficits related to impacts on working memory as a
result of trauma including loss of parent(s) which may have resulted in
disruption to schooling and lack of parental support for reading at home
(Gibbs et al., 2019). Another study, whose primary study factor was
health effects in children from wildfires used an existing cohort of
>6000 children, 86% aged 6 to 7 years old in California
(Künzli et al., 2006). Exposure was measured both subjectively and
objectively in two analytical models. Although academic achievement was
not assessed, the authors found that missed school attendance was
associated with increased respiratory symptoms especially in children
with an asthma diagnosis (21.7% compared to 11.55%) (Künzli et al.,
2006). School closures is a common outcome of wildfire, and in 2018 in
California more than a million children were impacted by school closures
due to wildfires (Chalupka and Anderko, 2019).
During the bushfire event in New South Wales, Australia of 2019, a
natural experiment was conducted in the context of an ongoing cluster
randomized controlled trial of a school-based physical activity
intervention in children aged 8 to10 years old (del Pozo Cruz et al.,
2021). During the bushfire event the schools received targeted health
advice from the government health department to avoid all outdoor
physical activity. Exposure was measured using air quality data
measurements located near the schools participating in the trial during
the bushfire, and physical activity was measured using accelerometers
that the children were wearing for the trial. The authors reported a
minimal impact on physical activity up to a ‘tipping point’ in air
quality when it was rated as ‘very poor’ and the physical activity
dropped sharply. This suggests that the public health advice did not
change children’s activity and was largely ignored until the air quality
reached very high pollution levels. Further, if air quality is poor for
an extended period due to wildfires, it may result in reduced physical
activity and children may have detrimental impacts on health outcomes.
The impact on infant feeding has also been assessed through the pathway
of exposure to emergency or disaster. A study from Canada of 115 women
with infants or young children up to 36 months were recruited through
social media after being evacuated during the 2016 Fort McMurray
wildfire. They reported a 30% reduction in breastfeeding before versus
after the evacuation (\(x^{2}\)=29.62, P=<0.001) (DeYoung et
al., 2018). Responses from participants suggested that many factors
resulted in reduced breastfeeding, including stress, perception of
reduced milk supply and lack of privacy.
Summary of childhood
studies
The impact of wildfire exposure on child health is an emerging area for
research investigation. Wildfire smoke exposure is a unique composition
of toxic chemicals that presents uniquely with shorter durations subject
to climate and geography resulting in unpredictable exposure patterns
(Black et al., 2017b). The effect on children is a research gap that
needs investigation, and the current body of evidence is a mix of
methodology and quality and has many of the same methodological issues
to the pregnancy literature, that make drawing meaningful and
generalizable conclusions difficult. Population-level data with accurate
exposure assessment, inclusion of important health outcomes, especially
those with children as the focus are needed. Evidence examined here
suggest that respiratory disease, either symptoms or exacerbation of
disease is a confirmed consequence from wildfire exposure in children,
as is increased health-service use including hospitalizations. Evidence
from ambient sources of PM2.5 not reviewed in this
chapter has more extensive investigation and more definitive results,
and the adult research as discussed in earlier chapters is also more
advanced and extensive, and suggests that there is much more to learn in
relation to the impact on children.