Ambient air pollution, as a risk factor for both acute and chronic diseases such as lung cancer, heart diseases, and acute respiratory infections, poses serious health threats to the world population [1
]. It is well established that both long-term [2
] and short-term [3
] exposure to ambient air pollution impact mortality, in particular cardiovascular and respiratory mortality. Although the deleterious effects of air pollution have been demonstrated in many Western countries [6
], there are fewer comparable studies conducted in Asia [9
]. Meanwhile, air quality remains a major problem in Asian cities, with effects on cardiovascular and respiratory mortality noted in time-series data [9
], along with significant public health implications [17
Seasonal exposure to transboundary haze is a major environmental health problem in Singapore, a heavily urbanized South-east Asian city-state [18
]. The Meteorological Service Singapore (MSS), which operates under the National Environment Agency (NEA), has established a robust system of monitoring stations across the city-state to provide round-the-clock surveillance of air quality. This surveillance capability creates opportunities for researchers to investigate the health effects associated with air pollution. The haze problem in Singapore generally coincides with the dry season from July to September, when the southwest monsoon shifts the haze resulting from forest fires caused by fire-fallow cultivation in neighboring countries toward Singapore [20
]. During periods of haze exposure, there is increased atmospheric loading of particulate matter and other aerosolized pollutants [21
]. Previous studies in Singapore suggest that during periods of haze, outpatient attendances for haze-related conditions such as respiratory tract illness rise [22
] and inpatient admissions increase [24
]. Time-series studies have demonstrated an association between haze exposure and acute ischemic stroke [25
], acute myocardial infarction [19
] as well as cardiac arrest [11
]. Studies in similar urbanized Asian settings have demonstrated associations between particulate matter air pollution and short-term mortality [15
]. We were, thus interested in investigating the association between short-term mortality (within 10 days of exposure) and ambient air pollution in our local context.
In this study, we examined the effect of air pollution on mortality in Singapore and demonstrated a significant association between an increase in PSI and mortality. In the short-term (lag = 7 days), PSI in the very unhealthy range appeared to have an adverse effect on mortality, although the effect was not statistically significant. In the longer term (lag >7 days), PSI in the unhealthy range similarly did not appear to have an effect on mortality. This could be attributed to the harvesting effect whereby PSI affected mainly a pool of frail individuals, and the depletion of this pool led to the reduction of potential deaths a few days later, thereby reducing the impact from PSI temporarily [44
]. On the other hand, the trend towards a reduced IRR at lag = 10 days, rather than a clear protective effect, is only suggestive but not conclusive of harvesting as an explanation. Reactive organizational-level protection measures, such as increased utilization of air filtration devices and more stringent limits on outdoor exposure for employed personnel, may also require time to take effect.
This is, to our knowledge, the first study linking the Southeast Asian haze to mortality in Singapore. While other studies in the region have documented increased morbidity associated with transboundary haze episodes, particularly for cardiovascular and respiratory morbidity, studies focusing on mortality are lacking [46
]. One study in neighboring Malaysia [47
] also demonstrated adverse associations between all-cause mortality and respiratory-specific mortality, and air pollution encountered during transboundary haze episodes. However, their study did not demonstrate a harvesting effect, perhaps because of the shorter lag times (up to seven days) studied [47
Our findings of a significant association between air pollution and mortality mostly corroborated the findings of other studies which used different measures of air quality and research designs, conducted in Australia [48
], Europe [50
], and Asia [14
]. Interestingly, Yin and colleagues [14
] found the highest risk of mortality when the lag was 0 compared to longer lags (up to six days), with no association between air pollution and mortality when lags were > 2 days. This is in contrast to our finding of the higher risk of mortality at lag 7 when PSI was in the very unhealthy range, although the association was similarly not significant.
The association between air pollution and mortality found in our study could possibly be attributed to an intermix of upper and lower respiratory tract infection and inflammation, allergic and hyperimmune reactions, oncologic implications, cardiovascular diseases, and distributed complications throughout the body, as mechanisms contributing to the degradation of health. In our previous studies, we similarly found an elevated risk of stroke [25
], acute myocardial infarction [19
], and out-of-hospital cardiac arrest [11
] after exposure to unhealthy PSI ranges. Notably, airborne particulates are classified as Group 1 carcinogens by the WHO (World Health Organization) and IARC (International Agency for Research on Cancer), due to their ability to penetrate deep into the lungs and bloodstreams unfiltered; while the PM10
particulates encompassed within the PSI metric are medically dangerous as thoracic and respirable particles, their presence might reasonably be correlated with submicron particles of even greater penetration. While the current study aims to investigate all-cause mortality as a way of providing highly general statistics between airborne pollution and population health, greater specific insights into the roles of these individual potential causes may be gained via a future cause-specific study.
4.1. Study Strengths
The strengths of the present study include high-quality death count data obtained from a national registry, which lends confidence in the complete data capture. Also, exposure data was directly measured by meteorological stations spread across the studied city-state of Singapore (and not obtained via modeling). Additionally, the conditional Poisson regression model used in this study accounted for overdispersion and autocorrelation in the time-dependent count’s data [36
], hence suppressing regression inaccuracies [51
] and the distributed lag non-linear model accounted for longer-term lag of up to 10 days.
4.2. Future Work
In this ecological study, we have demonstrated an association between PSI and mortality; however, it does not prove a causative relationship. Furthermore, as characteristic of data from death registries, there was no patient-level data available for subgroup analyses to identify susceptible subpopulations. A future cause-specific study with patient-level data can provide greater insights into susceptible subpopulations and the roles of individual potential causes of mortality.
As data of individual pollutant concentrations were not available, this limits the ability to relate the risk of mortality with each pollutant. However, the use of PSI is more pragmatic for the purpose of interpretation, informing policies and formulating public health messages as our results have shown.