RESEARCH ARTICLE

 

Household air pollution exposure and respiratory health outcomes: a narrative review update of the South African epidemiological evidence

 

 

Busisiwe Shezi^I; Caradee Y Wright^{II, III, *}

^IEnvironment and Health Research Unit, South African Medical Research Council, Durban, South Africa, busisiwe.shezi@mrc.ac.za
^IIEnvironment and Health Research Unit, South African Medical Research Council, Pretoria, South Africa
^IIIDepartment of Geography, Geo-informatics and Meteorology, University of Pretoria, Pretoria, South Africa, cwright@mrc.ac.za

 

 

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ABSTRACT

One of the greatest threats to public health is personal exposure to air pollution from indoor sources. The impact of air pollution on mortality and morbidity globally and in South Africa is large and places a burden on healthcare systems for treatment and care of air pollution-related diseases. Household air pollution (HAP) exposure attributed to the burning of solid fuels for cooking and heating is associated with several adverse health impacts including impacts on the respiratory system. The researchers sought to update the South African evidence on HAP exposure and respiratory health outcomes from 2005. Our quasi-systematic review produced 27 eligible studies, however, only four of these studies considered measures of both HAP exposure and respiratory health outcomes. While all of the studies that were reviewed show evidence of the serious problem of HAP and possible association with negative health outcomes in South Africa, no studies provided critically important information for South Africa, namely, local estimates of relative risks that may be applied in burden of disease studies and concentration response functions for criteria pollutants. Almost all of the studies that were reviewed were cross-sectional, observational studies. To strengthen the evidence of HAP exposure-health outcome impacts on respiratory health, researchers need to pursue studies such as cohort, time-series and randomised intervention trials, among other study designs. South African and other researchers working in this field need to work together and take a leap towards a new era of epidemiological research that uses more sophisticated methods and analyses to provide the best possible evidence. This evidence may then be used with greater confidence to motivate for policy-making, contribute to international processes such as for guideline development, and ultimately strengthen the evidence for design of interventions that will reduce HAP and the burden of disease associated with exposure to HAP in South Africa.

Keywords: environmental health, air quality, household emissions

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Introduction

While episodes of unusually high air pollution attract attention and public health concern, the greatest damage to public health is associated with long-term exposure to air pollution (HEI 2017). Outdoor and indoor personal exposure to air pollution, combined, comprise the largest environmental risk factor for mortality, responsible for 6.4 million deaths in 2015 (11% of global deaths) (Cohen et al. 2017).

The costs of air pollution in Africa are high - estimated at around USD450 billion in 2013 (Roy 2016). The economic impacts include life years lost, increased healthcare (and subsequent demand on government) and lost worker productivity due to air pollution impacts on health.

Globally, epidemiological studies and systemic reviews have shown associations between exposure to household air pollution (HAP) and a variety of diseases and symptoms (Jedrychowski et al. 2017; Tanaka et al. 2012; Koo et al. 2011; Pope et al. 2010). The indoor environment represents an important microenvironment in which people spend approximately 90% of their time each day (WHO 2014a).

According to the World Health Organization (WHO), HAP is responsible for more than 1.6 million annual deaths globally, and 2.7% of the global burden of disease (WHO 2014b). HAP is reported to increase irritation of the airways, coughing, irregular heartbeat, difficulty breathing and premature death in people with heart and lung disease (Gurley et al. 2013; Laumbach and Kipen, 2012; Ritz and Wilhelm, 2008). Exposure further worsens existing respiratory diseases such as bronchitis, cardiovascular disease and emphysema (Laumbach and Kipen, 2012; Fisk et al. 2010; Rinne et al. 2006). Exposure to solid fuel burning indoors has also been associated with tuberculosis (TB) (Jafta et al. 2015; Lin et al. 2014), cataract (Ravilla et al. 2016) and adverse birth outcomes (Wylie et al. 2017; Pope et al. 2010).

HAP is derived from multiple indoor sources (Colbeck and Nasir 2010) varying from one building to another and depending on fuels used for heating and cooking, smoking habits and use of a wide variety of consumer products and building materials (Shezi et al. 2017; Jafta et al. 2017; Tanaka et al. 2012; Verma et al. 2010). This is also influenced by time activity patterns i.e. cooking period, number of meals cooked, single or multiple fuel use and number of cigarettes smoked and further compounded by confounding variables such as outdoor air pollution sources near the homes (Shezi et al. 2017; Colbeck and Nazir 2010). In developing countries, the most significant indoor air quality issue is exposure to pollutants released during combustion of solid fuels used for cooking and heating in the home (Wylie et al. 2017; Pope et al. 2010).

The measured mean concentrations of the HAP vary depending on the sources, for example, average particulate matter with an aerodynamic diameter of 2.5 (PM₂₅) concentration in households using solid fuels has been reported to range from 133.5 ug/m³ to 670 ug/m³ (Balakrishnan et al., 2015; Balakrishnan et al., 2013; Clark et al., 2010), while the carbon monoxide (CO) average concentrations have been reported to range from 2.7 ppm to 14.3 ppm. The average PM₂₅ concentrations in households using cleaner fuels have been reported to range from 10 ug/m³ to 38 Ug/m³ (Shezi et al. 2017; Tunno et al., 2015; Evans et al. 2000). Studies conducted in South Africa have also reported high mean concentrations in households using both clean and dirty fuels (Jafta et al. 2017; Shezi et al. 2017; Wernecke et al 2015; Rollin et al. 2004) which exceed the guidelines set out by the WHO for indoor air quality in households; South African indoor air quality guidelines pertaining to household air pollution are under review and have not yet been promulgated.

South Africa is a middle-income country burdened by poverty and inequality where many South Africans are exposed to biomass fuels used for cooking and heating indoors. Many families live in close proximity to industrial areas and major roads (Albers et al. 2015; StatsSA 2012; Norman 2007) where outdoor pollutants may also be transported indoors (Colbeck and Nasir 2010). Vulnerable groups such as people who are elderly, women, young children, people with pre-existing diseases and people living in poverty are more susceptible to air pollution health impacts (Barnes 2014).

The WHO comparative risk assessment determined the mortality burden and Disability Adjusted Life Year (DALYs) due to HAP. HAP associated with household solid fuel use for cooking and heating caused 0.5% of all deaths in South Africa in 2000 (uncertainty 0.3% -0.6%) (Norman et al., 2010). More than 10 years ago, Wichmann and Voyi (2005) published a review of the air pollution and health epidemiological studies in South Africa, focussing on methodological issues and the need for quantitative intervention studies. In 2014, Barnes (2014) described behavioural change studies for reduction of HAP in developing countries, calling for more rigorous study design and interventions grounded in behavioural change theory. In recent years, anecdotal evidence suggests that studies on the impact of HAP and associated respiratory health outcomes has been growing in South Africa (Barnes et al., 2009). In this narrative review, evidence from the recently published South African studies that considered HAP exposure and associated respiratory health outcomes to augment our current knowledge is collated. An attempt to identify research gaps and suggest directions for further studies is made.

 

Review methods

A quasi-systematic (i.e. following the guidelines for systematic review but with slight differences in methods to accommodate all available evidence) review of the South African evidence on HAP (term group 1) and associated respiratory health outcomes (term group 2) was conducted using the PRISMA guidelines (Moher et al. 2009). PubMed, Web of Science, Science Direct and Google Scholar were searched for studies with full text in English, published between 2005 and 2017. The term groups listed in Table 1 were used for the separate searches and in various combinations. The reference lists of included papers were searched to ensure that no studies were omitted.

To be eligible, studies had to have been carried out in South Africa. All epidemiological study designs including cross-sectional studies, cohort studies, longitudinal studies, case-cross-over studies, intervention studies etc. were eligible. All studies that provided estimates of HAP exposure concentrations (by indicator, proxy or actual measurements) as well as respiratory health outcomes found to be associated with HAP exposure were included. We noted whether the correlate and health outcome was measured at the level of the individual, household or community. The review was not restricted by defining a minimal study sample size and studies of all sample sizes were included.

 

Results

After removing all ineligible articles, mostly since they were studies unrelated to South Africa, our searches using both formal methods (described above) and informal means (such as personal communication with researchers) produced a total of 27 articles. Of this total, only 4 studies measured HAP exposure (mainly by proxy/indicators of air pollution exposure) and respiratory health outcomes. Ten studies assessed HAP but no respiratory health outcome(s) and the remaining 13 studies did not measure HAP, instead used ambient AQ monitoring station data (or other) for exposure assessment.

Table 2 describes in brief the four studies that included HAP exposure monitoring and associated respiratory health outcomes by type of study, setting, sample size, methods, exposure information and outcome results. None of the studies measured HAP, instead, they used indicators or proxies for exposure, such as presence of environmental tobacco smoke (ETS) and fuel used for cooking and / heating in the home. Elf et al. (2017) used passive samplers to measure ETS. All of the studies were cross-sectional epidemiological study designs and the largest sample size was ~ 3 000 schoolchildren (Shirinde et al., 2014) with the other three studies having sample sizes of less than 1 000 individuals (no sample size calculations were provided). One study did not set out to measure HAP but rather focussed on ETS exposure.

Inter-comparison of the findings of the studies is difficult due to the heterogeneity in the study design, target population and health outcomes of interest. Both Albers et al. (2015) and Shirinde et al. (2014) found that among schoolchildren (albeit of different ages) there was an association between respiratory outcomes (most notably wheeze) and use of non-electrical heating sources. Elf et al. (2017) and du Preez et al. (2011) consider TB as a health endpoint and ETS and solid fuel use, and ETS, respectively, in relation to TB.

Tables 3 and 4 summarise the remaining studies that the researchers reviewed as part of this review exercise. These studies did not meet the study inclusion criteria; however, they do provide useful information on the levels of various air pollutants in the indoor and outdoor environments of different parts of South Africa. Since they did not mention respiratory health outcomes, the researchers do not discuss them in detail here, but included them as a reference for future research.

Of the twenty-three studies listed in Table 3 and 4, two studies were review articles, six studies used questionnaires and interviews which are methods of assessment that are based on self-report and recall. Either indoor and / or outdoor pollutants were measured in 15 studies and some of the studies measured multiple pollutants. Particulate matter with an aerodynamic diameter of 10 (PM₁₀) was measured in twelve studies while PM₂₅ and particulate matter with an aerodynamic diameter of 4 (PM₄) were measured in four and two studies, respectively. Two studies measured respiratory particulate matter (RPM). NO₂ was measured in five studies; NO_x in one study and volatile organic compounds were also measured in one study. Three studies measured CO and seven studies measured sulphur dioxide (SO₂). Ozone (O₃) was measured in one study and total reduced sulphur was also measured in one study. Indoor undisturbed dust samples were undertaken in one study to analyse for dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyl-dichloroethylene (DDE) and dichlorodiphenyl-dichloroethane (DDD), while settled dust and airborne fungal sampling was also conducted in one study.

 

Discussion

Few published studies conducted in South Africa in recent years have sought to show associations between HAP and respiratory health outcomes. All of the studies that did so used cross-sectional study design. This is unfortunate since most international systematic reviews for HAP exposure and health outcome association only use data from cohort, time series, long-term panel (longitudinal) and case-crossover studies. Our studies are therefore not contributing to international evidence on this important topic, nor are they providing reliable evidence that is generalizable to others parts of South Africa. While cross-sectional studies are typically less expensive and easier to implement compared to other epidemiological study designs, the data produced from cross-sectional studies is not as useful and the lack of randomisation, among other shortfalls, prohibits generalisation. Researchers need to work towards larger, more complex epidemiological studies and also use of existing, high quality (where possible) data in South Africa that will help to address the problem of HAP and respiratory health. While it will likely cost more, other study designs beside cross-sectional studies, will provide opportunities for HAP monitoring over substantial periods of time to provide exposure information that can then be associated with health outcomes, preferably diagnosed by a health professional (and not self-reported or parent-reported) for more precise results, in a more meaningful way.

It is also important to account for potential biases and critical confounders, including age, sex, and individual socio-economic status, among others, when planning a HAP and respiratory health study, and also when reporting study results as failure to control for confounding variables can lead to erroneous associations between HAP and respiratory outcomes.

While the reviewed studies predominantly report on combustion generated variables (type of fuel used for cooking or heating and active or passive smoking) with PM, CO and NO₂, being products of incomplete combustions, other sources of indoor air pollutants not necessarily emitted by incomplete combustion such as building materials, ventilation characteristics and cleaning agents may not be ignored.

While, exposure data misclassification and validity of exposure are often overlooked or underestimated and not critically discussed (Wichmann and Voyi, 2005), the lack of direct exposure measurement such as the use of home monitors and personal monitors present results that are in some part questionable.

None of the studies reviewed provided concentration response functions for the criteria air pollutants in South Africa (DEA, 2009). This point was made by Wichmann and Voyi (2005) and it still holds true in 2017. This is still a major gap in our knowledge in both South Africa and on the continent. South African researcher continue to use the international literature and WHO evidence, without making contribution to this important body of knowledge.

Another shortfall is that only one study reviewed in this exercise was an intervention study (under real life conditions). The WHO calls for the support of research that is driven by interventions and searching actively for solutions, in particular for urban settings, in relation to air pollution and health (WHO 2014). In South Africa, research partnerships and consortia may assist to create large research teams to lead intervention studies to address HAP and adverse health impacts in areas of greatest concern.

Our study had some limitations. There were very few studies to critically review hence the researchers opted to describe them descriptively instead. Wichmann and Voyi (2005) provided a critical synthesis of the evidence in this field up to 2005. The authors did not find a substantial number of studies to add to that body of literature beyond 2005. The authors set out to find studies that had measured HAP and simultaneously measured respiratory health outcomes. The authors may have not identified all studies relevant to the review inclusion criteria, although they tried to avoid this by searching the literature regularly and speaking with South African researchers in the air pollution and health fields. The authors did not consider mortality as an end-point. Wichmann and Voyi (2006) found that exposure to cooking and heating smoke from polluting fuels was significantly associated with 1-59-month mortality, after controlling for mother's age at birth, water source, asset index and household crowdedness (RR=1.95; 95% CI=1.04, 3.68).

 

Conclusions

The South African studies on HAP and respiratory health outcomes do provide some evidence of the serious impacts of HAP and especially the use of solid fuels in the home on respiratory health in the country, but the studies are few and limited. South African and other researchers working in this field need to work together and take a leap towards a new era of epidemiological research that uses sophisticated methods and analyses to provide the best possible evidence. This evidence may then be used with greater confidence to, for example, motivate for policy-making, contribute to international systematic reviews for guideline development and other purposes, and ultimately strengthen interventions that will reduce HAP and the burden of disease associated with exposure to HAP in South Africa.

 

Acknowledgements

We thank MA Oosthuizen for assisting with data collection. B Shezi and CY Wright receive research funding support from the South African Medical Research Council. CY Wright receives funding support from the National Research Foundation.

 

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Received: 18 March 2018
Reviewed: 10 June 2018
Accepted: 14 June 2018