Re: Aircraft noise and cardiovascular disease near Heathrow airport in London: small area study
Kolstad and colleagues cite largely negative results of a large Danish study of occupational noise exposure, hypertension (based on registry data)  and stroke  to question whether results of our study on aircraft noise and cardiovascular disease may reflect confounding rather than causal associations. Occupational studies are subject to the ‘healthy worker’ bias – for example, those prone to ill-health related to noise exposure are more likely to leave or never start working in occupations with high noise exposures; while adjustment was made for early leavers in the study some residual bias may remain. Further, the widespread use of hearing protection devices in noisy occupations is likely to influence this relationship, which the authors were not able to adjust for. A meta-analysis of 15 occupational studies published 1991-2007 (i.e. some relating to exposures prior to widespread use of hearing protection devices) showed associations between high occupational noise exposure and hypertension and ECG abnormalities , which this later study did not.
Findings for cortisol in long-term exposure studies are more mixed. Kolstad and colleagues did not find an association with morning salivary cortisol in their occupational study . However, the Hypertension and Exposure to Environmental Noise around Airports (HYENA) population-based study found salivary cortisol was raised in women but not men in association with 24 hour average aircraft noise exposure .
Published research into potential health effects of environmental noise exposure is expanding. For example, a recently published long-term prospective cohort study with careful consideration of confounders including smoking found long-term exposure to fine particulate air pollution (PM10) and night-time traffic noise were both independently associated with subclinical atherosclerosis . A recently published experimental field study of short-term effects of aircraft noise exposure at night  found noise was associated with increases in plasma adrenaline (but not cortisol), decreases in pulse transit time (a measure of arterial stiffness) and a dose-dependent effect on endothelial dysfunction.
Ongoing evaluation of evidence from a range of studies in different settings with different designs including occupational, cohort and experimental such as these is needed to help clarify the possible relationship between noise exposure and cardiovascular disease.
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Competing interests: The authors declare: financial support for the submitted work through the funding of the UK Small Area Health Statistics Unit by Public Health England as part of the MRC-PHE Centre for Environment and Health, funded also by the UK Medical Research Council; financial support from the European Network for Noise and Health (ENNAH), EU FP7 grant No 226442; PE acknowledges support from the National Institute for Health Research (NIHR) Biomedical Research Centre based at Imperial College Healthcare NHS Trust and Imperial College London; PE is an NIHR Senior Investigator; ALH declares consultancy fees from AECOM as part of a Defra report on health effects of environmental noise; ALH declares a Greenpeace membership but has not received any money from the organisation nor been involved in campaigns; nor other relationships or activities that could appear to have influenced the submitted work.