Cardiorespiratory hospitalizations associated with smoke exposure during the 1997 Southeast Asian forest fires

Abstract

We investigated the cardiorespiratory health effects of smoke exposure from the 1997 Southeast Asian Forest Fires among persons who were hospitalized in the region of Kuching, Malaysia. We selected admissions to seven hospitals in the Kuching region from a database of all hospital admissions in the state of Sarawak during January 1, 1995 and December 31, 1998. For several cardiorespiratory disease classifications we used Holt-Winters time-series analyses to determine whether the total number of monthly hospitalizations during the forest fire period (August 1 to October 31, 1997), or post-fire period (November 1, 1997 to December 31, 1997) exceeded forecasted estimates established from a historical baseline period of January 1, 1995 to July 31, 1997. We also identified age-specific cohorts of persons whose members were admitted for specific cardiorespiratory problems during January 1 to July 31 of each year (1995–1997). We compared Kaplan-Meier survival curves of time to first readmission for the 1997 cohorts (exposed to the forest fire smoke) with the survival curves for the 1995 and 1996 cohorts (not exposed, pre-fire cohorts). The time-series analyses indicated that statistically significant fire-related increases were observed in respiratory hospitalizations, specifically those for chronic obstructive pulmonary disease (COPD) and asthma. The survival analyses indicated that persons over age 65 years with previous hospital admissions for any cause (${\chi }_{1\mathrm{df}}^2=5.98$, $p=0.015$), any cardiorespiratory disease (${\chi }_{1\mathrm{df}}^2=5.3$, $p=0.02$), any respiratory disease (${\chi }_{1\mathrm{df}}^2=7.8$, $p=0.005$), or COPD (${\chi }_{1\mathrm{df}}^2=3.9$, $p=0.047$), were significantly more likely to be rehospitalized during the follow-up period in 1997 than during the follow-up periods in the pre-fire years of 1995 or 1996. The survival functions of the exposed cohorts resumed similar trajectories to unexposed cohorts during the post-fire period of November 1, 1997 to December 31, 1998. Communities exposed to forest fire smoke during the Southeast Asian forest fires of 1997 experienced short-term increases in cardiorespiratory hospitalizations. When an air quality emergency is anticipated, persons over age 65 with histories of respiratory hospitalizations should be preidentified from existing hospitalization records and given priority access to interventions.

Introduction

In 1997, uncontrolled forest fires burned in the Indonesian states of Kalimantan and Sumatra. The fires, in combination with a severe drought, produced a regional air pollution episode that affected Malaysia, Singapore, Thailand, Brunei and the Philippines. During a continuous two-month period, an estimated 20 million people were exposed to ambient concentrations of particulate matter (PM₁₀) that exceeded the US EPA's 24 h National Ambient Air Quality Standard of 150 μg/m³ (Brauer et al., 2001). On September 22, 1997, 24-h PM₁₀ concentrations peaked at 852 μg/m³ in Kuching, the capital of the state of Sarawak, Malaysia. This peak concentration exceeded the US EPA's 24-h hazardous level of 500 μg/m³.

As population expansion into wildland environments continues, human exposures to forest fire smoke are likely to increase (US Centers for Disease Control and Prevention, 1999, US Centers for Disease Control and Prevention, 2001, US Centers for Disease Control and Prevention, 2002; Ward, 1999). A limited number of epidemiologic investigations suggest that population exposures to wood smoke are associated with increased self-reporting of respiratory symptoms (US Centers for Disease Control and Prevention, 2002; Larson and Koenig, 1994; Shusterman et al., 1993) and increased emergency department admissions for chronic obstructive pulmonary disease (COPD), bronchitis, asthma, and chest pain (US Centers for Disease Control and Prevention, 1999, US Centers for Disease Control and Prevention, 2001; Mott et al., 2002; Duclos et al., 1990). However, these health endpoints are influenced by individual perceptions and decisions to seek medical care. Thus they are somewhat subjective and can increase with the psychosomatic stress that is often associated with air pollution emergencies. Population densities of exposed communities have not commonly been large enough to produce sample sizes sufficient to assess epidemiologic relationships between poor air quality and more objective and severe indicators of morbidity such as hospitalizations for cardiorespiratory problems (US Centers for Disease Control and Prevention, 2001).

When dispensing intervention resources into smoke-exposed communities, public health professionals tend to assume that older persons, and persons with COPD, are most susceptible to the adverse health effects of forest fire smoke exposure (Mott et al., 2002). However, an assessment of cardiorespiratory health effects in cohorts of exposed and unexposed persons of different ages and disease histories has not been undertaken. The systematic identification of those most susceptible to forest fire smoke exposures remains necessary for the development of science-based recommendations for public outreach and targeted intervention.

In this study we investigate the cardiorespiratory health effects of the 1997 Southeast Asian forest fires among persons who were hospitalized at any of seven hospitals in the region of Kuching, Malaysia between January 1, 1995 and December 31, 1998. Specifically, this study has three objectives:

• To determine whether there was an increase in total hospitalizations, cardiovascular hospitalizations, and respiratory hospitalizations during the months of the forest fires in 1997 relative to the non-fire years of 1995 and 1996, adjusting for seasonal variation in admissions.

• To assess whether cohorts of persons with previous hospital admissions for cardiorespiratory diseases were more likely to be readmitted for the same condition during the forest fires of 1997 than during the same months during pre-fire years of 1995 and 1996.

• To assess the duration of any longer term post-fire health effects (in the form of longer term increases in re-hospitalization rates among persons with a history of disease) in cohorts of persons exposed to the forest fire smoke.

Hospitalization data provided by the Sarawak Health Department provided a unique opportunity to undertake this investigation as: (a) the population density of exposed persons was sufficient to permit cohort analyses of hospitalizations, and (b) the absence of significant industrial pollution permitted an independent assessment of the health effects of forest fire smoke exposure (Sastry, 2002).