Household Coal Use and Lung Cancer Systematic Review and Meta-analysis

Lung cancer risk and solid fuel smoke exposure: a systematic review and meta-analysis

European Respiratory Journal 2012 40: 1228-1237; DOI: x.1183/09031936.00099511

Abstract

The aim of this systematic review was to quantify the impact of biomass fuel and coal apply on lung cancer and to explore reasons for heterogeneity in the reported effect sizes.

A systematic review of primary studies reporting the human relationship between solid fuel employ and lung cancer was carried out, based on pre-defined criteria. Studies that dealt with misreckoning factors were used in the meta-analysis. Fuel types, smoking, country, cancer cell type and sex were considered in sub-group analyses. Publication bias and heterogeneity were estimated.

The pooled effect approximate for coal smoke as a lung carcinogen (OR 1.82, 95% CI 1.threescore–2.06) was greater than that from biomass smoke (OR i.fifty, 95% CI i.17–1.94). The risk of lung cancer from solid fuel use was greater in females (OR 1.81, 95% CI i.54–2.12) compared to males (OR i.xvi, 95% CI 0.79–ane.69). The pooled result estimates were 2.33 (95% CI 1.72–3.17) for adenocarcinoma, 3.58 (1.58–8.12) for squamous cell carcinoma and 1.57 (one.38–i.lxxx) for tumours of unspecified cell type.

These findings advise that in-home burning of both coal and biomass is consistently associated with an increased run a risk of lung cancer.

• Adenocarcinoma
• biomass fuel
• coal
• indoor air pollution
• squamous cell carcinoma

Lung cancer is one of the leading causes of mortality accounting for ane.three million deaths annually worldwide [i]. While smoking is the major run a risk factor, 25% of cases are not owing to tobacco apply [2]. Epidemiological studies accept shown that globally while lung cancer in never-smokers is consistently more common in females than in males, geographical variations are substantial [2]. In eastern and southern Asia, up to 83% of female lung cancer cases are never-smokers, compared to 15% in the Us [two]. In developing countries an estimated ii.4 billion people (seventy%) use biomass (wood, charcoal, crop residues or dung) or coal, collectively known every bit solid fuels, for cooking and heating [iii]. Emissions from combustion of solid fuels have been shown to have loftier concentrations of polycyclic effluvious hydrocarbons (PAHs), benzo[a]pyrene and particulate matter with a diameter of 2.v μm or less, which in plow have been associated with high rates of lung cancer [2].

Recently, indoor emissions from household combustion of coal and biomass (generally forest) have been classified equally carcinogenic (Group one) and probably carcinogenic (Group 2A) to humans [4]. However, information on the magnitude of lung cancer risk and the histological sub-type of lung cancer associated with solid fuel use are express. In the literature, 4 meta-analyses were identified, only three [5–7] were limited to studies conducted in China and i [8] focused only on coal utilise. A contempo paper included a pooled estimate from several countries, just data were restricted to studies from an international consortium [ix].

In this meta-analysis we reviewed papers from all countries and calculated pooled estimates of the association of the apply of solid fuels and lung cancer. We investigated whether these effects were influenced differently past other factors, notably the types of fuel used, smoking (including ecology tobacco smoke (ETS)) and study location. We also looked at whether there was a pattern of association between fume exposure and lung cancer histological sub-type.

METHODS

Papers published from January 1980 to October 2010 were identified through a systematic literature search in Ovid Medline, EMBASE and Google Scholar. Search terms used for the initial search on exposure were "biomass", "biofuel", "organic fuel", "blackness smoke", "wood", "indoor air pollution", "carbon monoxide", "respirable dust", "solid fuel", "dung", "charcoal", "ingather rest", and outcomes were "carcinogen", "lung neoplasm", "adenoma", "adenocarcinoma", "squamous carcinoma", "carcinoma", "lung cancer" and "cancer". The manufactures obtained by using different exposure search terms were combined using "OR" and the same was washed for outcomes. The combining term "AND" was used to combine the article obtained for exposure and outcome. References in each of the identified papers were screened for any articles that were not identified in the original search. There was no restriction on language in the original search merely articles in English and Chinese were retained for inclusion in the meta-assay. The search was carried out by two authors (P.H. Arya and O.P. Kurmi).

Study selection

All potentially relevant articles were reviewed. Selection criteria were identified and defined by all co-authors. For studies to be part of the review and meta-analysis they had to see the criteria listed in table ane. Most studies considered were those in which cases had cytological/histological findings alongside radiological confirmation. However, a minority of the studies where the assessment technique was not stated were still included in the review. No limitations were prepare for the historic period of participants in the studies or for the definition of exposure to solid fuels.

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Table one– Inclusion criteria for meta-analysis

Data extraction

Selection of studies was undertaken at each phase past ii authors (P.H. Arya and O.P. Kurmi) for studies written in English language and one writer for studies written in Chinese (Thousand-B.H. Lam). Disagreements were settled by consensus. All data were extracted by 2 authors (P.H. Arya and K-B.H. Lam) independently and uncertainties were discussed with all authors. We used the Newcastle-Ottawa Quality Assessment Scale to assess the quality of the studies [ten]. A pre-divers course was then used to extract information from selected studies nether the following headings: author; journal; twelvemonth; state of study; system/funding trunk; blazon of fuel considered; study design; smoking (blazon, measure and assessment technique); sample size; indoor air pollution exposure assessment; master outcome (type and assessment of outcome); upshot size (relative take chances or odds ratio (OR) and the associated 95% confidence intervals (CI) and p-values); and possible confounding factors considered.

Statistical analysis

Initially all studies were pooled and a sensitivity assay was carried out to assess the touch on of methodological business organisation by grouping them into unlike sub-groups, which include fuel types (biomass/mixed fuel/coal), sexual practice (female person only/male only/male and female), cancer histological sub-type (unspecified/adenocarcinoma/squamous carcinoma), adjustment for smoking (aye/non-smokers), aligning for ETS exposure (yes/no), study design (population/hospital based), sample size (median >368/≤368), study location (China/Taiwan/India/other), year when study was conducted (2000 onwards/prior to 2000), year of publication (2000 onwards/prior to 2000), language of publication (Chinese/English), Newcastle-Ottawa score (median >6/≤half-dozen), and the quality of exposure cess based on the Newcastle-Ottawa criteria (1/2/three stars). The natural logarithm of odds ratio and the associated standard fault were used to judge the effect size of all studies and the sub-groups. Inside-grouping heterogeneity was evaluated using Q tests and/or I² statistics. Heterogeneity between different studies was visually explored using Galbraith plots, and sources of heterogeneity were systematically examined by meta-regression. We used random effects models as at that place was meaning heterogeneity on Q tests (p<0.05) and/or Iⁱⁱ statistic value >50%. Begg's funnel plot and Egger's exam were used to assess publication bias [11]. All analyses were performed in STATA (version 11; STATA, College Station, TX, United states).

RESULTS

The initial search revealed eleven,398 manufactures of which 2,012 duplicates and 7,908 irrelevant papers were removed by screening the titles. The abstracts of the remaining ane,478 papers were reviewed and 203 were selected for total paper review, of which 51 papers were related to lung cancer and solid fuel apply (fig. 1). 28 studies (tabular array two) were included in the meta-analysis, the other 23 papers were excluded either considering of failure to run into the inclusion criteria or because data were unusable, or both (tabular array S1). The results presented are from 12,419 cases and 34,609 controls.

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Table two– Summary of studies included in the meta-assay

Effect estimates

The pooled effect estimate size was obtained using the random effect model because of heterogeneity across studies (Q=statistic 107.30, degrees of freedom=40, p<0.001; Iⁱⁱ=62.seven%; τ²=0.081, Z=7.99, p<0.001). The pooled OR was 1.70 (95% CI 1.50–i.94) for all studies.

Sub-group analyses were performed using random consequence models. The values related to biomass, mixed fuel and coal were OR i.50 (95% CI i.17–1.94), OR 1.13 (0.52–2.46) and OR 1.82 (ane.60–2.06), respectively (fig. ii). Forest plots are presented in figures S1–S8. Coal contributed 68.viii% to the pooled effect sizes of lung cancer followed by biomass (19.8%) and mixed fuel (eleven.5%). The associated risk for females was greater compared to that for males (p=0.034) (tabular array 3). The greater take chances observed in the Chinese publications compared to those in English (p=0.006) remained later adjusting for potential confounders including types of fuel used, sexual activity, smoking and quality of the study assessed by the Newcastle-Ottawa score. The aforementioned tendency was found in both smoking and nonsmoking participants.

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Table 3– Sub-grouping analyses of lung cancer risk associated with the utilize of solid fuels

Studies were then stratified according to the type of fuel used and then by various sub-groups (tabular array four). No significant heterogeneity was observed in the different strata for studies related to the exposure to biomass smoke only heterogeneity amidst hospital-based studies approached significance (I²=54.iii%, p=0.053). Even so, there was significant heterogeneity among studies with coal smoke exposure in relation to squamous cell carcinoma (I^two=61.2%, p=0.035), unspecified types of lung cancer (I²=38.1%, p=0.047), females only (I²=45.5%, p=0.043), population-based (I²=sixty.four%, p=0.001) and hospital-based studies (I²=43.four%, p=0.008), and those with sample size ≤368 (Iⁱⁱ=49.three%, p=0.019).

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Table 4– Sub-group analyses of lung cancer take chances co-ordinate to fuel blazon

Of the 28 studies included in the meta-analysis, 14 collected data on ETS exposure and only seven made adjustments for ETS. Even more surprising was the fact that only three out of seven of the female simply studies that measured ETS actually adjusted for ETS. Pooled upshot estimates from studies that adjusted for ETS (OR one.28, 95% CI 0.91–i.fourscore) were significantly lower (p=0.034) compared to those that did not (OR 1.91, 95% CI ane.65–2.22).

The studies with poor quality, especially in the exposure assessment, as measured by the Newcastle-Ottawa score, tend to study greater effect size (tables 3, 4 and S7).

Publication bias

Funnel plots suggested potential publication bias for the biomass (fig. S7) and coal smoke (fig. S8) studies. Yet, Egger'south test showed substantial publication bias only in coal smoke studies (bias=1.04, p=0.016) (fig. S10), which disappeared when two outlying studies were removed (bias=0.76, p=0.093) [26, 29]. The pooled effect estimate (OR 1.64, 95% CI 1.45–one.86) was slightly attenuated afterwards excluding the two outliers.

Heterogeneity by meta-regression

Heterogeneity was initially explored by graphical display (Galbraith plot) (fig. S11 for biomass and fig. S12 for coal), then by meta-regression to assess contributions by sex, histological sub-type, smoking, aligning for ETS exposure, sample size, study location, year in which the study was carried out, year of publication, and language of publication. In studies of biomass smoke exposure, significant merely modest heterogeneity was observed in sex (coefficient= -0.253, p=0.025), although there was a nonsignificant heterogeneity in lung cancer histology (coefficient=0.636, p=0.057). However, in studies of coal fume exposure, language of publication (coefficient=0.308, p=0.032) and histology (coefficient=0.273, p=0.058) had like magnitude of heterogeneity, although the latter was non statistically significant. We did not find evidence of heterogeneity (p=0.116) between the studies of better quality (Newcastle-Ottawa score >6) and poorer quality (≤half dozen).

DISCUSSION

This meta-assay included studies conducted in Prc, Taiwan, Japan, India, Mexico, Morocco, USA and Canada, as well every bit a study carried out jointly in seven European countries (Czechia, Hungary, Poland, Romania, Russian federation, Slovakia and the UK). The pooled issue estimates that the take a chance of lung cancer amid users of solid fuels is 70% (95% CI l–94%) higher than not-users.

The magnitude of association between coal use and lung cancer (OR 1.82, 95% CI 1.threescore–2.06) was greatest followed by biomass (predominantly forest, OR i.50, 95% CI 1.17–1.94) and mixed fuel (OR 1.xiii, 95% CI 0.52–2.46), although the differences were not statistically meaning. The higher risk of lung cancer in coal users was non surprising as combustion products obtained from in-home coal burning incorporate a range of Group i carcinogenic PAHs [four]. While there is sufficient bear witness to propose exposure to biomass fume is a run a risk cistron for chronic obstructive pulmonary affliction (COPD) in adults [forty] and acute respiratory infection in children [41], the International Agency for Research on Cancer (IARC) has classified combustion products from biomass (primarily wood) as probable human lung carcinogens (Group 2A), citing in that location was "express prove" in humans and experimental animals [iv]. The pooled effect size obtained from studies using population-based controls (carrying 56% weight; OR i.83, 95% CI 1.51–2.21) was similar to that using infirmary-based controls (39% weight; OR one.63, 95% CI 1.34–ane.97). Among the 28 studies included, 2 population-based studies [sixteen, 38] and ii hospital-based studies [33, 36] did not find an increased gamble of lung cancer. Of these, three were related to biomass use [16, 33, 38] and the other to coal utilise [36] supporting the IARC notion that the evidence of the carcinogenicity of biomass fume is however not conclusive.

The association betwixt lung cancer and solid fuel employ persisted even after stratifying for sex, fuel types, smoking and study location. The duration of exposure in most of the studies was not clearly defined and there was marked variability in reported exposure intensity across studies but the number of studies were besides pocket-sized to make up one's mind any dose–response human relationship. Of the 28 studies included in this meta-assay, ii studies scored the maximum of three stars on the Newcastle-Ottawa criteria for exposure whereas 18 studies scored two and viii studies scored only one star. The studies with the highest quality in exposure assessment take lower effect sizes suggesting that misclassification and residual misreckoning might exist operating, thereby inflating the risk approximate. Users of biomass often switch from one type of biomass to another. A detailed history on the type, duration and intensity of fuel use (such as average number of hours exposed) must exist gathered in future studies to better estimate the risks from detail biomass fuels as combustion products from different types of biomass called-for have variable toxicity [42].

Cigarette smoking has been widely accepted as the main contributory factor to lung cancer worldwide [43, 44]. We excluded two papers on the basis that smoking had non been immune for in the adventure estimates [45, 46], and all studies included in this review take either adjusted for smoking or studied a population of nonsmokers. A contempo meta-analysis included effect estimates from Chinese studies that did not adjust for smoking [8]. The extent of misreckoning is, yet, difficult to predict. While it is accepted that self-reported smoking history is the best that tin can be accomplished when considering life-long smoking details, objective measurement of smoking, such as salivary cotinine, is becoming more than easily usable in field studies and provides information on current smoking, which may, to a certain extent, aid reduce exposure misclassification. This is particularly the case for females from countries who hesitate to admit to smoking for the fearfulness of marginalisation.

Although one-half of the studies included in the meta-analysis measured ETS, merely 25% of them presented data with adjusted ETS exposure. In studies that did, the pooled effect size (OR 1.47, 95% CI i.xiii–i.91) was smaller than (only not statistically significant, p=0.230) those that did not (OR 1.74, 95% CI 1.60–one.89). In females the pooled effect estimate with adapted ETS was significantly lower compared to non-adjusted ETS suggesting the overall pooled effect estimate, particularly in females, might be lower than presented hither. Only one study out of 8 related to biomass fume exposure adjusted for ETS and had an effect size higher than those that were not adjusted for ETS. Thus, ambiguity regarding the combined effect of smoking, combustion products of solid fuels and ETS exposure still prevails and hereafter studies need to address this issue, particularly in females from Asian sub-continent as they are highly probable to be exposed to ETS. There is evidence from occupational studies that smoking and some occupational exposures (east.g. asbestos and PAHs) have a multiplicative rather than an additive event on lung cancer take a chance [47, 48] and information technology is, therefore, possible that such a potentiating issue may be seen with respect to smoke from solid fuel burning, specially that from coal.

Females in developing countries do most of the cooking and, thus, are more likely to exist exposed to indoor air pollution than males. The pooled outcome size shows that the risk of lung cancer is greater in females (OR i.81, 95% CI i.54–2.12) compared to males (OR 1.16, 95% CI 0.79–1.69), similar to that reported in a limited earlier meta-assay for females but (OR 1.83, 95% CI 0.62–5.41) [7]. Many published meta-analyses reported data for males and females combined. In this study, the pooled outcome size for both sexes was one.93 (95% CI 1.53–2.44), smaller than that reported by Zhao et al. [7] (OR ii.66, 96% CI 1.39–5.07), probably because the latter was obtained from the coal using population in Red china. The pooled effect size in our study would accept been reduced to i.80 (95% CI 1.46–2.22) if the two studies with effect sizes of 24.34 (95% CI two.97–199.48) [29] and xiv.10 (95% CI ane.37–145.61) [17] were excluded.

The pooled event gauge in studies published in the Chinese linguistic communication (OR 2.16, 95% CI 1.81–2.59) was significantly greater (p=0.006) than studies published in English. When scrutinising the Chinese papers, we establish a consistently large effect size. While the event could be real, as Chinese papers focused on the coal using Chinese population and that coal has been recognised by the IARC as a carcinogen, this raises a business organization on the overall quality of the enquiry published in Chinese journals.

Table 5 presents the main findings from previously published meta-analyses (including our study). Over sixty% of these (five out of eight) included studies either from China or the Chinese population only and examined only the effects of coal apply. In contrast, the current meta-analysis presents the pooled results from various geographical regions, and has investigated the effects of biomass and coal exposure separately. In addition, nosotros have specified in our inclusion criteria that but those studies that have adapted for smoking or used a nonsmoking sample would be included, therefore, minimising potential confounding from smoking.

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Table 5– Pooled outcome estimates from previous meta-analyses on solid fuel use and lung cancer

To our noesis, this is the beginning assessment of whether solid fuel smoke is associated with specific histological sub-types. Prison cell type was reported in eight papers but the criteria for histological classification were non provided. The pooled outcome size for squamous cell carcinoma (OR iii.58, 95% CI i.58–viii.12) was greatest followed by adenocarcinoma (OR 2.33, 95% CI 1.72–3.17) and unspecified type of lung cancer (OR 1.57, 95% CI one.38–one.80). Squamous cell lung cancer is more commonly associated with cigarette smoking [52] although reported series of lung cancers have recently shown an increase in the proportion of adenocarcinoma which cannot simply be attributed to changes in classification/grading [53]. If cell type reflects different carcinogenic properties of different exposures and then time to come studies studying the risk of lung cancer from solid fuel would benefit by classifying the types of lung cancer by fuel blazon.

Most of the studies included in this meta-analysis are from People's republic of china where coal is the main fuel. The pooled effect size in Taiwan (three studies: OR two.34, 95% CI i.39–three.94) is greater than that in Prc (17 studies: OR i.77, 95% CI 1.57–two.00). None of the studies included from Communist china and Taiwan have looked at the association betwixt coal type and lung cancer adventure. Nonetheless, evidence from a community with high lung cancer bloodshed in China suggested that bituminous or "smoky" coal, with a loftier volatile content (23.1%), was more carcinogenic compared to smokeless coal which contains relatively high sulfur (1.9%) but low volatiles (13.eight%) [54]. Further investigation [54, 55] concluded that compared to woods and smokeless coal, smoky coal contains more methylated PAH compounds, nitrogen heterocyclic compounds and dibenzo[a,l]pyrene, a potent carcinogen with the highest mutagenic activeness in mice.

Almost studies did non mensurate exposure quantitatively. Understanding the shape of the dose–response bend has been a challenge for a range of outcomes arising from biomass smoke exposure (eastward.thou. COPD [xl] and acute respiratory infections in children [41]), but is crucial in determining to what extent exposures would demand to be reduced in club to confer a pregnant health benefit. Notwithstanding, measuring current exposures may simply partially reflect historical exposures, fifty-fifty though in many areas where solid fuel is burnt, exercise and, therefore, exposures have probably remained similar for decades. Nevertheless, if formal quantification of exposures tin can be undertaken in hereafter studies this will provide relevant data to address this issue.

Our results suggested an chemical element of publication bias which could be due to fewer positive studies being rejected and more positive studies, some with flawed methodology, beingness accepted. The meta-regression showed that there was significant heterogeneity among studies reporting different types of lung cancer.

Conclusion

Our meta-analysis suggested that coal is highly associated with lung cancer compared to other types of biomass. The adventure was greater in females and in People's republic of china which could be because Chinese females used coal. Time to come studies demand to expect at objective measurements of smoking and besides the carcinogenic potential of different coal subtypes to explain some of the variability seen in the risk estimates.

Footnotes

• This article has supplementary textile available from world wide web.erj.ersjournals.com

• Statement of Interest

  None alleged.

• Received June x, 2011.
• Accepted Feb 26, 2012.

• ©ERS 2012

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