Perspectives in Household Air Pollution Research: Who Will Benefit from Interventions?
Curr Envir Health Rpt
Perspectives in Household Air Pollution Research: Who Will Benefit from Interventions?
Maggie L. Clark
Jennifer L. Peel
0 Department of Environmental and Radiological Health Sciences, Colorado State University , 1681 Campus Delivery, Fort Collins, CO 80523-1681 , USA
Household air pollution from solid fuel combustion in inefficient and poorly vented cookstoves is estimated to be responsible for 3.9 million premature deaths per year and 4.8 % of the global burden of disease, making it the third leading risk factor for morbidity and mortality worldwide. Despite increasing recognition surrounding this global environmental health problem, much remains to be elucidated regarding exposure response relationships, particularly among potentially susceptible population subgroups. Given that many of the communities most affected by household air pollution exposures also experience elevated exposures to poverty, psychosocial stressors, other environmental pollutants, and comorbid conditions, research needs to correctly specify risks due to these potentially interacting risk factors. Although suggestive evidence exists for differential improvements in health following reductions in ambient air pollution concentrations among specific subgroups, the question remains as to who will benefit and to what extent from efforts to reduce exposures to emissions from household solid fuel combustion. The ability to know what to expect from cookstove interventions and to accurately describe the presence of distinct subgroup responses is crucial to reduce uncertainty and to encourage policy makers to enact change.
Biomass; Cookstoves; Household air pollution; Health; Differential susceptibility; Interventions
In recent years there has been increasing recognition of the
staggering public health and environmental impact of
emissions from household solid fuel combustion utilized by
nearly three billion people worldwide for cooking, heating,
and lighting needs [
1, 2, 3
]. This form of energy
utilization, which consists of burning solid fuels (e.g., wood, crop
residue, animal dung, coal, etc.) over an open fire or in a
traditional cooking stove, is very inefficient, leading to the
formation of toxic combustion by-products like carbon
monoxide [CO] and particulate matter [PM], among many
]. These exposures are disproportionately
experienced by women and children due to domestic roles
typically encountered in least-developed or developing
]. Average daily PM concentrations in homes
burning solid fuels can range from 300 to 3,000 μg/m3 [
substantially greater than the World Health Organization
guidelines for ambient and indoor air quality intended to
protect public health (e.g., the guidelines for daily mean
PM2.5 [PM with an aerodynamic diameter less than or
equal to 2.5 microns] and PM10 are 25 μg/m3 and 50 μg/
m3, respectively) .
Often characterized by large variability and uncertainty due
to differences in factors such as stove use and time-activity
patterns, household room configuration and ventilation, fuel
type and moisture, weather, and instrument error [
to household air pollution has been linked to serious global
health impacts, both acute and chronic [
]. Air pollution from
household solid fuel combustion is the third leading risk factor
for morbidity and mortality globally, responsible for an
estimated 3.9 million premature deaths per year and
approximately 4.8 % of disability-adjusted life years [
]. The burden due
to household air pollution far surpasses the burden of other
environmental risk factors such as unsafe water and
insufficient sanitation . Despite increasing recognition, the
attention and resources devoted to this problem do not match
the severity of the issue. As a result, multiple knowledge gaps
and critical research priorities for household air pollution and
related health impacts have been identified. Here, we present a
summary of recent health effects reviews (see Table 1) and
discuss knowledge gaps not previously emphasized that are
critical to characterizing the exposure-response relationship
and answering the questions of who will benefit and to what
extent from efforts to reduce exposures to emissions from
household fuel combustion.
**A noted limitation of summarizing risk estimates in reviews and meta-analyses of household air pollution is the heterogeneous nature of the exposure
measures used across individual studies; “exposed” versus “unexposed” categories are often based on factors related to the fuel type for cooking (e.g.,
biomass versus a “clean” fuel), stove type, rural-urban comparisons, outdoor versus indoor cooking, and time exposed to biomass fuel combustion [
The State of the Science: Household Air Pollution and Health Impacts
Our understanding of the disease burden from solid fuel
combustion is still relatively limited; however, consistently
and substantially elevated risks have been demonstrated for
acute lower respiratory infections in children [
obstructive pulmonary disease (COPD) [
], lung cancer
], and cataracts [
]. Although an exhaustive review of
the health effects of household air pollution is beyond the
scope of this manuscript, Table 1 provides a summary of
recent systematic reviews on the primary health outcomes
considered by the 2010 Comparative Risk Assessment for
Household Air Pollution from the use of solid fuels for
]. Detailed descriptions of the systemic reviews
and meta-analyses for each health endpoint considered were
provided by Smith et al . Additionally, convincing evidence
exists for adverse cardiovascular-related endpoints [
information on the exposure-response function determined
from other sources of combustion-related air pollution
exposure (e.g., ambient air pollution, secondhand smoke, and
active tobacco smoke) was extrapolated when developing
the Comparative Risk Assessment for household air pollution
and hard cardiovascular disease endpoints [
evidence also suggests associations with adverse pregnancy
], tuberculosis [
], and cognitive
]. Furthermore, recent publications have identified
important gaps in knowledge focusing on the shape of the
exposure-response curves [
], the need for improved
exposure assessment, particularly challenging when long-term
exposures are of interest [
], as well as the implications that solid
fuel cookstove emissions have on climate-level impacts [
Ideally, the solution to household air pollution exposures is
to transition households to cleaner fuels (e.g., liquid petroleum
gas, electricity). The use of cleaner fuels tends to occur
naturally with economic development and such efforts to
encourage and promote modern energy access are underway in some
parts of the world. However, the economic situation in many
regions implies that a substantial proportion of the world’s
population, those unable to afford and/or access modern fuels,
will continue to burn solid fuels for many decades. Therefore,
it is critical to find economically feasible and culturally
appropriate alternatives to the traditional solid fuel stove. Recent
evidence suggests that the introduction of cleaner stoves is
capable of improving health. For example, investigators
evaluating the large-scale Chinese National Improved Stoves
Program (NISP) reported improvements in several respiratory
disease endpoints [
]. Additionally, the first randomized
controlled trial evaluating the health impact of a chimney
stove in Guatemala demonstrated a reduction in severe
childhood pneumonia, although the effect was not statistically
significant when all pneumonia types were considered
]. Beyond respiratory outcomes, the intervention in
Guatemala also resulted in lower blood pressure levels among
]. Notwithstanding these encouraging results,
many efforts to disseminate and sustain cleaner-burning
cookstove technologies across the developing world have had
limited success. Indeed, many of the stove intervention
projects conducted have failed—some due to poor stove design
and/or performance in the field (i.e., the exposure reduction
achieved is still high compared to the WHO guidelines) and
others due to cultural issues (i.e., lack of adoption of new stove
technology). A recent review of studies measuring behavior
change surrounding the dissemination of various cookstove
technologies worldwide highlighted the need to characterize
the most important factors (e.g., individual, household, and
societal) necessary to promote the adoption and sustained
proper use of clean fuels and/or cleaner-burning biomass
Who Benefits from Cleaner-burning Cookstove
Interventions and To What Extent?
Although the evidence regarding the adverse health impacts of
household air pollution is growing, uncertainty remains as to
the health improvements expected as a result of interventions
introducing cleaner burning stoves. Evidence that certain
characteristics confer increased susceptibility to the adverse
effects associated with exposures to air pollution is mounting,
especially for ambient air pollution [
]. Here, we define
susceptibility as individual- and population-level
characteristics that increase the risk of air pollution-related health effects
in a population. Susceptibility may, therefore, indicate the
presence of different exposure-response relationships among
different populations (i.e., given the same level of air pollution
exposure, some populations will experience greater health
effects than others), or it may also refer to a characteristic of
a population that increases the likelihood or opportunity for
greater exposure to certain pollutants (sometimes referred to
as vulnerability) [
]. Factors that have been observed to
increase susceptibility to air pollution exposures include age,
sex, genetics, underlying health, obesity, diet, smoking status,
socioeconomic status, and psychosocial stressors [
Much of the evidence regarding susceptibility is supported
by the ambient air pollution literature, conducted primarily in
urban areas of industrialized countries. However, this issue is
gaining momentum within the household air pollution
literature and evidence for effect modification now exists,
particularly for cardiovascular-related effects. For example, the
relationship between integrated 24-hour personal PM2.5
concentrations and elevated systolic and diastolic blood pressure was
stronger in women >50 years of age among a population of
traditional stove users in China [
]. Similarly, a slight
increase in the odds ratio for solid fuel use and hypertension was
observed among those ≥40 years of age as compared to those
<40 years of age in a population-based study of over 14,000
Chinese men and women (p-value for interaction= 0.34) [
The same investigators reported stronger evidence of effect
modification by sex and smoking status when evaluating
effects of solid fuel use on hypertension and coronary heart
disease; elevated risks were observed among women
(pvalues for interaction= 0.02 and 0.32, respectively) and among
never-smokers (p-values for interaction= 0.02 and 0.02,
]. Finally, effects on systolic blood pressure
were stronger among obese women in Nicaragua for both
48-hour indoor PM2.5 and indoor CO (p-value for
interaction= 0.04 and 0.002, respectively) [
]. In addition to this
compelling, initial evidence of differential susceptibility,
further evaluation of other environmental, comorbid, social, and
genetic factors will likely prove critical for determining an
accurate picture of the magnitude of health effects due to
household air pollution [
19, 45, 46
Several investigators have hypothesized that those
individuals who are more susceptible to the adverse effects of air
pollution exposure may also be the groups that benefit most
from efforts to reduce air pollution levels (e.g., traffic
reduction plans, industrial facility closings, indoor air filter
], yet this question has largely been ignored
in the cookstove field. Valid assessments of the true
exposureresponse relationships among various subpopulations are
necessary to inform a more accurate estimate of the global burden
of disease attributed to cookstove smoke, an identified
research gap needed to convince governments and policy
makers to enact interventions . Evidence regarding who
benefits from improved air quality is limited and inconsistent.
It is not known whether larger predicted benefits among
certain subpopulations are due to differences in greater
relative improvements associated with air pollution reductions
(i.e., different exposure-response functions experienced by
the subgroups) or differences in absolute improvements
because of poorer baseline health status, which may be
independent of air pollution [
Figure 1 is a simplistic example of the problem that may
arise if investigators do not take into account that only a
segment of their study population is able to experience a
health benefit resulting from a cookstove intervention. Here,
the “responders”, or those benefiting most from the
intervention, represent about a third of the total population, and both
the responders and the non-responders have the same mean
health measure at baseline (top panel). If the mean change in
health response due to the intervention is calculated assuming
the population is homogenous (i.e., ignoring that the subgroup
of responders exists), then only a small, likely statistically
non-significant improvement would be observed (a);
however, the responders’ improvement after the intervention may be
meaningful (b) and should be described to inform public
health officials as well as future studies. More complex
scenarios likely exist in real-world settings. For example, the
responders may start with poorer health status at baseline
and also experience a greater response to the reduction in
Furthermore, although the scenario depicted in Figure 1
represents one study population that includes a proportion of
responders, defining and identifying responders versus
nonresponders is also relevant for comparing results across study
populations. This phenomenon might, in part, explain the
heterogeneity in effects observed across studies evaluating
distinct populations. For example, Clark et al. [
] did not
observe a substantial mean reduction in systolic blood
pressure among the entire population receiving a stove
intervention in Nicaragua, which included women aged 11–80 years,
with a mean age of 35 years; however, the mean reduction
among those 40 years and older (-5.9 mmHg [95 % CI: -11.3,
0.4]) was comparable to the within-person average change
(3.1 mmHg [95 % CI: -5.3, -0.8]) observed among the entire
Guatemalan study population with a mean age of 53 years
after a similar stove intervention [
]. Furthermore, although
reduced occurrence of ST-segment depression was associated
with the same stove intervention in Guatemala, no evidence of
effect modification by age or body mass index was observed
]. It will be important to consider the restricted age range as
well as the somewhat limited variability in body mass index
(mean = 24.3 kg/m2 ± 3.0 [standard deviation] at baseline)
when comparing these results to future cookstove
interventions. It is also important to note that the factors influencing
the observed benefits of cookstove interventions are likely not
the same for all health endpoints.
Evidence from the ambient air pollution literature supports
the importance of investigating factors that may influence the
observed benefits of interventions that reduce exposure to air
pollution. The implementation of traffic-reduction policies in
two large European city-centers resulted in observed health
benefits for different socio-economic segments of the
respective populations. In London, Tonne et al. reported greater life
expectancy benefits among the low socio-economic group
]. In contrast, Cesaroni et al. reported greater health
benefits of traffic-related air pollution reductions among the higher
socio-economic group in Rome [
]. The observed
differences between the studies are due to differences in the
residential make-up of the two cities; those with lower
socioeconomic status tend to reside in the most polluted, central
areas of London, whereas those with higher socio-economic
status are more likely to live near the city-center in Rome [
]. Therefore, although differences in baseline health across
socio-economic groups were considered, those benefiting
most were those that experienced the greater reductions in
traffic-related pollution following the policy changes. In other
words, the scenarios described focused primarily on
differences in exposure opportunity by the “susceptibility” factor
(here, socio-economic status). Additionally, since Tonne et al.
did not apply different exposure-response functions to the
socio-economic groups, the authors stated that the resulting
benefits to the lower socio-economic group in London may
have been underestimated .
Challenges also arise when predicting health benefits for
various subgroups of the population if the risk factor that
defines the responder subpopulation has a nonlinear or
threshold effect on the health endpoint of interest [
example, improvements were not observed in symptoms
following an allergen-reducing indoor air quality intervention
in a public housing complex among asthmatic children
reporting exposure to a psychosocial risk factor (e.g., fear of
]. Similarly, improved air quality was associated
with a slower rate of decline in several lung function
parameters among non-obese but not among obese participants in a
study of Swiss adults [
]. Counter to initial hypotheses,
those with factors typically thought to confer increased
susceptibility to the adverse effects of air pollution exposures
(e.g., psychosocial factors and obesity) did not experience
greater benefits following reductions in air pollution
exposures. These types of scenarios suggest that threshold or
saturation effects occurring among those highly exposed to
a secondary risk factor may overshadow or even negate any
potential improvement resulting from a reduction in air
]. This phenomenon may be a particularly important
consideration to discern when estimating benefits from
cookstove interventions since the target populations are typically
affected by several co-occurring risk factors (e.g., low
socioeconomic status, poor nutrition, exposures to pesticides),
many of which are increasing rapidly in some developing
countries (e.g., obesity, diabetes). Particular attention needs to
be paid to the ranges of these co-occurring risk factors within
and across study populations to obtain valid estimates of
exposure-response (i.e., for cumulative and/or interacting
risks) and to inform the interpretations of observed
differences in results across studies.
Another obstacle to the valid assessment of exposure-response
relationships, and hence a more accurate estimate of predicted
health benefits resulting from stove interventions and
dissemination efforts, rests within our inability to identify enablers
and barriers to cleaner-burning cookstove uptake and
sustained use over time [
32, 57, 58
]. Behavior change is
difficult to predict and measure in any public health scenario.
The adoption of new cooking technologies may depend on a
wide range of factors, including cultural, financial,
geographical, familial, and individual factors [
acceptance of stoves within populations has been commonly
]. In the simplest scenario, these factors
confer differences in the opportunity for exposure to air
pollution emissions (e.g., older women may not adopt the
cookstove as readily as younger women). In this situation,
the exposure-response relationships may not be different for
various age groups, but younger women will be more likely to
experience health improvements than older women because
their exposures have been reduced more.
In a more complex scenario, the factors that influence the
degree to which a new cookstove is adopted and sustainably
used may be the same factors that lead to heterogeneous
exposure-response relationships. Although this phenomenon
has not been evaluated thoroughly in cookstove research,
lessons can be gleaned from other fields. For example,
educational attainment is often associated with mortality. Level of
education may also play a role in behaviors surrounding
smoking, which may mediate the relationship with mortality
while simultaneously acting as an effect modifier of the
relationship between education and mortality [
]. In order to
develop a complete picture it is critical to address both the
differential opportunities for exposure to air pollution (and in
the case of interventions, factors that may lead to greater
behavior changes surrounding the adoption of new stoves)
as well as factors that may confer increased responsiveness or
different exposure-response curves across categories, as the
two scenarios may not be mutually exclusive [
Although a limited amount of work has been conducted to
characterize those who benefit most from interventions
intended to improve air quality (e.g., traffic reduction
schemes, indoor air filtration), much remains to be elucidated
regarding this question for cookstove interventions. Accurate
identification of factors that modify the relationships between
reductions in cookstove-related air pollution exposures and
improved health endpoints, including the correct identification
of the shape of the responses and potential for threshold
effects, is not just an academic exercise but information
critical to making progress on this global health problem. The
global disease burden estimates assume homogeneous
exposure-response relationships, yet many of the communities
affected by household air pollution exposures also experience
a multitude of additional chronic disease risk factors. Many of
these co-occurring risk factors are already known or
hypothesized to modify the effects of air pollution exposures from
various combustion-related sources. The ability to know what
to expect from cookstove interventions (i.e., to accurately
describe the presence of the subgroup response, as simplified
in Figure 1, as being a meaningful shift in health
improvement) is crucial to reducing scientific uncertainty and to
encourage policy makers to enact change. In order to gain
the biggest advances in health improvements, or what may be
considered sufficient health improvements in cost-benefit
scenarios, answers to these questions may demonstrate the
Figure 1 Hypothetical
population distribution of a health
endpoint measured at baseline
and following a cookstove
intervention. The black line
represents the entire target
population (mean change in
health represented by [A]) and the
gray line represents the
proportion of the population that
responds to the reduction in
household air pollution (mean
change in health represented by
[B]). (Adapted from: Weiss B and
Bellinger DC. 2006. Social
ecology of children’s
vulnerability to environmental
pollutants. Environmental Health
importance of developing co-interventions aimed at reducing
more than one risk factor. Despite the challenges and
knowledge gaps discussed here and elsewhere, large-scale and
ambitious initiatives for the dissemination of cleaner-burning
stoves are underway or planned [
1, 49, 63
international efforts are proving to be a critical impetus for the scientific
research sector to provide credible evidence on the
exposureresponse relationships among potentially susceptible
population subgroups. If the relationships between these additional
risk factors and household air pollution are correctly specified,
we will gain a much better understanding of the health patterns
observed in these vulnerable communities—those often
experiencing elevated exposures to poverty, psychosocial
stressors, other environmental pollutants, and comorbid
conditions, all of which have been implicated within the
multifactorial nature of the health conditions commonly associated
with household air pollution.
Acknowledgments Maggie L. Clark reports grants from the National
Institutes of Health (K99ES022269).
Jennifer L. Peel reports grants from the National Institutes of Health
Compliance with Ethics Guidelines
Conflict of Interest Maggie L. Clark and Jennifer L. Peel declare that
they have no conflicts of interest.
Human and Animal Rights and Informed Consent This article does
not contain any studies with human or animal subjects performed by any
of the authors.
Papers of particular interest, published recently, have been
Of major importance
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