Using epidemiology to estimate the impact and burden of exposure to air pollutants.

This paper focuses on the use of results of epidemiological studies to quantify the effects on health, particularly on mortality, of long-term exposure to air pollutants. It introduces health impact assessment methods, used to predict the benefits that can be expected from implementation of interventions to reduce emissions of pollutants. It also explains the estimation of annual mortality burdens attributable to current levels of pollution. Burden estimates are intended to meet the need to communicate the size of the effect of air pollution on public health to policy makers and others. The implications, for the interpretation of the estimates, of the assumptions and approximations underlying the methods are discussed. The paper starts with quantification based on results obtained from studies of the association of mortality risk with long-term average concentrations of particulate air pollution. It then tackles the additional methodological considerations that need to be addressed when also considering the mortality effects of other pollutants such as nitrogen dioxide (NO2). Finally, approaches that could be used to integrate morbidity and mortality endpoints in the same assessment are touched upon. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Lung cancer mortality and exposure to polycyclic aromatic hydrocarbons in British coke oven workers.

BACKGROUND: Workers on coke oven plants may be exposed to potentially carcinogenic polycyclic aromatic hydrocarbons (PAHs), particularly during work on the ovens tops. Two cohorts, employees of National Smokeless Fuels (NSF) and the British Steel Corporation (BSC) totalling more than 6,600 British coke plant workers employed in 1967, had been followed up to mid-1987 for mortality. Previous analyses suggested an excess in lung cancer risk of around 25%, or less when compared with Social Class IV ('partly skilled').Analyses based on internal comparisons within the cohorts identified statistical associations with estimates of individual exposures, up to the start of follow-up, to benzene-soluble materials (BSM), widely used as a metric for mixtures of PAHs. Some associations were also found with times spent in certain coke ovens jobs with specific exposure scenarios, but results were not consistent across the two cohorts and limitations in the exposure estimates were noted. The present study was designed to reanalyse the existing data on lung cancer mortality, incorporating revised and improved exposure estimates to BSM and to benzo[a]pyrene (B[a]P), including increments during the follow-up and a lag for latency. METHODS: Mean annual average concentrations of both BSM and B[a]P were estimated by analysis of variance (ANOVA) from concentration measurements at all NSF and six BSC plants, and summarised by job and plant, with a temporal trend (for the BSM only). These were combined with subjects' work histories, to produce exposure estimates in each year of follow-up, with a 10-year lag to allow for latency. Exposures to BSM and to B[a]P were sufficiently uncorrelated to permit analysis in relation to each variable separately.Lung cancer death risks during the follow-up were analysed in relation to the estimated time-dependent exposures, both continuous and grouped, using Cox regression models, with adjustment for age. RESULTS: Changing the exposure estimates changed the estimated relative risks compared with earlier results, but the new analyses showed no significant trends with continuous measures of exposure to either BSM or B[a]P, nor with time spent on ovens tops. Analyses with grouped exposures showed mixed results. Across all BSC plants, the relative risk coefficient for working 5 or more years on ovens tops, where the exposures were highest, was 1.81, which was statistically significant. However, results for those with 0-5 years on ovens tops did not suggest a trend; the evidence for an underlying relationship was thus suggestive but not strong. CONCLUSIONS: The new results are in line with previous findings; they show some signs consistent with an effect of coke ovens work on lung cancer risk, especially on ovens tops, but the preponderant absence of significant results, and the inconsistencies between results for NSF and BSC, highlight how little evidence there is in these data of any effect.

Using PM2.5 concentrations to estimate the health burden from solid fuel combustion, with application to Irish and Scottish homes.

BACKGROUND: This study estimates the potential population health burden from exposure to combustion-derived particulate air pollution in domestic settings in Ireland and Scotland. METHODS: The study focused on solid fuel combustion used for heating and the use of gas for cooking. PM2.5 (particulate matter with an aerodynamic diameter < 2.5 µm) was used as the pollutant mixture indicator. Measured PM2.5 concentrations in homes using solid fuels were adjusted for other sources of PM2.5 by subtracting PM2.5 concentrations in homes using gas for cooking but not solid fuel heating. Health burden was estimated for exposure indoors 6 pm - midnight, or all day (24-hour), by combining estimated attributable annual PM2.5 exposures with (i) selected epidemiological functions linking PM2.5 with mortality and morbidity (involving some re-scaling from PM10 to PM2.5, and adjustments 'translating' from concentrations to exposures) and (ii) on the current population exposed and background rates of morbidity and mortality. RESULTS: PM2.5 concentrations in coal and wood burning homes were similar to homes using gas for cooking, used here as a baseline (mean 24-hr PM2.5 concentrations 8.6 µg/m3) and so health impacts were not calculated. Concentrations of PM2.5 in homes using peat were higher (24-hr mean 15.6 µg/m3); however, health impacts were calculated for the exposed population in Ireland only; the proportion exposed in Scotland was very small. The assessment for winter evening exposure (estimated annual average increase of 2.11 µg/m3 over baseline) estimated 21 additional annual cases of all-cause mortality, 55 of chronic bronchitis, and 30,100 and 38,000 annual lower respiratory symptom days (including cough) and restricted activity days respectively. CONCLUSION: New methods for estimating the potential health burden of combustion-generated pollution from solid fuels in Irish and Scottish homes are provided. The methodology involves several approximations and uncertainties but is consistent with a wider movement towards quantifying risks in PM2.5 irrespective of source. Results show an effect of indoor smoke from using peat (but not wood or coal) for heating and cooking; but they do not suggest that this is a major public health issue.

Costs of occupational asthma in the UK.

OBJECTIVES: To estimate the social costs of occupational asthma in the UK. METHODS: A desk-top approach using cost-of-illness methodology was employed, defining direct and indirect lifetime costs for six scenarios: a male and a female worker each exposed to isocyanates, latex and biocides (eg, glutaraldehyde) or flour. The numbers of new cases annually in each industry were estimated from Survey of Work-related and Occupational Respiratory Disease (SWORD) data. The main outcome measure was the current value total working lifetime costs of new cases annually for each scenario. RESULTS: Assuming 209 new cases of occupational asthma in the six scenarios in the year 2003, the present value total lifetime costs were estimated to be £25.3-27.3 million (2004 prices). Grossing up for all estimated cases of occupational asthma in the UK in 2003, this came to £70-100 million. About 49% of these costs were borne by the individual, 48% by the state and 3% by the employer. CONCLUSIONS: The cost to society of occupational asthma in the UK is high. Given that the number of newly diagnosed cases is likely to be underestimated by at least one-third, these costs may be as large as £95-135 million. Each year a new stream of lifetime costs will be added as a newly diagnosed cohort is identified. Approaches to reduce the burden of occupational asthma have a strong economic justification. However, the economic burden falls on the state and the individual, not on the employer. The incentive for employers to act is thus weak.

The brave new world of lives sacrificed and saved, deaths attributed and avoided.

Attributable risks are routinely estimated in "risk-factor" epidemiology. Often, these risks are interpreted as the numbers of deaths caused by (or numbers of lives lost by) exposure to the factor of interest. It is then often surmised that removal of exposure will avoid deaths and save lives. This reasoning leads to confusion because of 2 underlying assumptions. One is that removal of the exposure will reduce permanently the annual number of deaths by the number attributed to the factor. In reality, deaths are merely postponed and lives are merely prolonged; estimating the effect of exposure on life expectancy is both more straightforward and of greater public health interest. The other misleading premise is that the deaths attributed to a certain risk factor can be identified. While such identification may be possible for certain immediate external causes of death (eg, accidents), it is not usually feasible for deaths attributed to factors that merely contribute to development of chronic disease and ultimately death. For such exposures, it is more reasonable to suggest that they contribute to death in all who are exposed - more so in some people than in others. Again, it is more appropriate to calculate the average loss of life expectancy associated with exposure from follow-up studies; the years of life lost by individuals who are exposed then varies around this average. The "real age" concept popularized in lifestyle Web sites and television programs may be more useful than calculations of the numbers of attributable deaths for communicating individual as well as public health risks associated with common environmental, occupational and lifestyle risk factors.