Review article
Association between fire smoke fine particulate matter and asthma-related outcomes: Systematic review and meta-analysis

https://doi.org/10.1016/j.envres.2019.108777Get rights and content

Abstract

Background

Asthma-related outcomes are regularly used by studies to investigate the association between human exposure to landscape fire smoke and health. Robust summary effect estimates are required to inform health protection policy for fire smoke exposure.

Objective

To conduct a systematic review and meta-analysis to estimate the association between short-term exposure to landscape fire smoke (LFS) fine particulate matter (PM2.5) and asthma-related outcomes.

Methods

We conducted a systematic review and meta-analysis following PRISMA guidelines. Four databases (PubMed, Medline, EMBASE and Scopus) and reference lists of recent fire smoke and health reviews were searched. The Newcastle-Ottawa Scale was used to evaluate the quality of case-crossover studies, and a previously validated quality assessment framework was used for observational studies lacking control groups. Publication bias was assessed using funnel plots and Egger's Test. The trim and fill method was used when there was evidence of publication bias. Sensitivity and influence analyses were conducted on all endpoints to test the robustness of estimates. Summary estimates were obtained for hospitalisations and emergency department (ED) visits. A descriptive analysis was conducted for physician visits, medication use, and salbutamol dispensations.

Results

From an initial 181 articles (after duplicate removal), 20 studies were included for quantitative assessment and descriptive synthesis. LFS PM2.5 levels were positively associated with asthma hospitalisations (RR = 1.06, 95% CI: 1.02–1.09) and emergency department visits (RR = 1.07, 95% CI: 1.04–1.09). Subgroup analyses found that females were more susceptible than males for ED visits, and that there was an increasing association by age groups for hospital admissions and ED visits. High heterogeneity between studies was observed, but results were robust to sensitivity analysis.

Conclusions

Females and all adults aged over 65 years appear to be the population groups most sensitive to asthma-related outcomes when exposed to LFS PM2.5. Overall, results were higher than those obtained for a typical PM2.5 mixture.

Introduction

Asthma is a diverse condition which generally involves chronic inflammation of the airways. It is defined by the Global Initiative for Asthma (GINA) (2018 p. 14) as “the history of respiratory symptoms such as wheeze, shortness of breath, chest tightness and cough that vary over time and in intensity, together with variable expiratory airflow limitation”. In 2016, it was estimated that the prevalence of asthma was more than 330 million people (GBD 2016 Disease and Injury Incidence and Prevalence Collaborators, 2017), and that this translated into more than 23 million disability adjusted life years (DALYs), positioning asthma in the 28th position among the leading causes of burden of disease (Global Asthma Network, 2018; Global Initiative for Asthma, 2018). Evidence shows that prevalence is higher in males during childhood but increases in females after puberty (Ferrante and La Grutta, 2018). In the case of older people, evidence is scarce, but shows that asthma is usually more severe and associated with other comorbidities such as chronic obstructive pulmonary disease (COPD) (Battaglia et al., 2016; Curto et al., 2019). Healthcare systems and society are stressed through increased hospitalisations, emergency department visits, medication dispensations, morbidity and mortality. Families and the workplace could be potentially economically affected due to loss of productivity (Global Initiative for Asthma, 2018). Ultimately, these impacts translate to high average per patient yearly costs (Nunes et al., 2017), such as the estimated EUR 1583 (in 2010 euros) for adults aged 30–54 years in Europe (Accordini et al., 2013) or the USD$ 3266 (in 2015 U.S. dollars) for the general population in the USA (Nurmagambetov et al., 2018).

Indoor and outdoor air pollution has been recognised as an important environmental risk factor associated with asthma (Beasley et al., 2015). It is estimated that globally between 5 and 10 million asthma emergency department visits could be attributable to fine particulate matter (PM2.5), representing 4–9% of the annual number of global visits (Anenberg et al., 2018). PM2.5 from landscape fire smoke (LFS) has been associated with a myriad of health effects, diverse in type and magnitude, ranging from all-cause mortality to respiratory hospital admissions and emergency department (ED) visits and increased use of medication for treating asthma and asthma-like symptoms (Adetona et al., 2016; Black et al., 2017; Cascio, 2018; Liu et al., 2015; Reid et al., 2016a). Asthma-related outcomes are amongst the most reported endpoints.

Health impact assessments, cost-effectiveness analyses, and cost-benefit analyses are commonly used methodologies for estimating burden of disease and health costs associated with air pollution and for analysing different pollution (exposure) reduction strategies. They are particularly useful when informing environmental and public health policies (Cárdaba Arranz et al. 2014; Martuzzi et al., 2003). Currently, there are no recommended concentration response functions (CRFs) for quantifying asthma-related outcomes attributable to particulate matter (PM) for conducting health impact assessments, except for the incidence of asthma symptoms in asthmatic children and daily mean PM10 (World Health Organization Regional Office for Europe, 2013). Furthermore, LFS PM2.5, which is composed of multiple chemicals produced during biomass combustion (Johnston et al., 2012), is different to a typical or multisource PM2.5 mixture which includes different chemicals produced by the combustion of multiple sources and types of fuels. Recent systematic reviews and meta-analyses have calculated pooled effect estimates for the short-term effect of PM2.5 on asthma-related hospital admissions and ED visits, identifying positive and stronger associations for males, children and elders (Fan et al., 2016; Lim et al., 2016; Zheng et al., 2015). Nevertheless, these studies were not focused on LFS or a smoke-dominated PM2.5 mixture, but rather on general multi-source air pollution. Previous studies have found that asthma-effects related to LFS PM are higher than non-LFS PM (Alman et al., 2016; DeFlorio-Barker et al., 2019; Gan et al., 2017; Johnston et al., 2014; Morgan et al., 2010; Reid et al., 2016b).

With an increasing expectation of more intense and frequent fires in the future, and an increasing use of prescribed burns for the reduction of wildfire risk, there is a constant need for evidence that will adequately help inform policy. The purpose of this study was to conduct a systematic review and meta-analysis of the associations between short-term exposure to LFS PM2.5 and asthma-related outcomes, including hospital admissions, emergency department visits, physician visits, ambulance dispatches, medication use, and salbutamol dispensations. When possible, we performed subgroup analyses to obtain summary estimates by age group, sex, lags, country, study design, and exposure method.

Section snippets

Methods

The PRISMA protocols for systematic reviews and meta-analysis (Moher et al., 2015) were used, and the protocol was registered in PROSPERO1 under registration ID CRD42018108767 at the beginning of the study.

Study characteristics

The search identified a total of 181 articles after duplicates were removed. After performing the title and abstract screening, 60 studies were assessed for full-text eligibility, out of which only 20 were included for the quantitative and descriptive synthesis (see Fig. 1).

Table 2 presents a summary of characteristics of studies included in the quantitative and descriptive analyses. Most studies (n = 15) estimated effects for hospital admissions, ED visits or both, and were done either in

Discussion

Our results suggest that short term exposure to LFS PM2.5 is positively associated with asthma-related outcomes, and that this association varies by sex, age group, country of study, and study design. A positive association was found for hospital admissions in all sexes and all ages, adults, and elders. Point estimates were higher for same day (lag 0) compared to 1,2, and 3 days after exposure, but positive in all situations. Results varied between countries (Australia and USA) and study design

Conclusion

Our systematic review and meta-analysis suggests that asthma-related hospital admissions and emergency department visits are positively associated with LFS PM2.5, and some groups such as women, adults, and elders are more affected than others. Hospital admissions results show that smoke effects could last for multiple days, while for ED visits effects tended to occur on the same day as exposure. Currently there are no recommended coefficients for asthma-related outcomes, apart from asthma

Authors contributions

NB, FJ, AP and GM participated in the design of the study and protocol. NB and FJ participated in the coordination. NB drafted the manuscript. NB and JH performed the study screening, selection and data extraction. NB and RT did the quality assessment. NB, FJ and GM defined the overall focus of the study. NB performed the statistical analysis. All authors critically reviewed the manuscript for its intellectual content, and approved the final manuscript.

Declaration of competing interest

The authors declare they have no actual or potential conflicts of interest.

Acknowledgements

We thank Breanna L. Alman, Frank Curriero, Ryan W. Gan, Ivan C. Hanigan, Sarah B. Henderson, Sumi Hoshiko, and George Le for providing us with their unpublished data. We thank The New South Wales Government's Department of Planning, Industry & Environment who provided funds to support this research via the Bushfire Risk Management Research Hub.

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    Sources of financial support: NB is supported by a Tasmania Graduate Research Scholarship, by Asthma Australia through a Top-up Scholarship, and by the New South Wales Bushfire Risk Management Research Hub through a Top-up Scholarship. JH was supported by an Australian Government Research Training Program (RTP) Scholarship through the University of Sydney and a Top-up Scholarship from the Centre for Air Pollution, energy and health Research (CAR).

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