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Year : 2009  |  Volume : 11  |  Issue : 44  |  Page : 161--168

Exposure-response relationship of the association between aircraft noise and the risk of hypertension

Wolfgang Babisch1, Irene van Kamp2,  
1 Department of Environmental Hygiene, Federal Environment Agency, Germany
2 Centre for Environmental Health Research, The National Institute for Public Health and Environmental Protection, Netherlands

Correspondence Address:
Wolfgang Babisch
Federal Environment Agency, Corrensplatz 1, 14195 Berlin
Germany

Abstract

Noise is a stressor that affects the autonomic nervous system and the endocrine system. Under conditions of chronic noise stress the cardiovascular system may adversely be affected. Epidemiological noise studies regarding the relationship between aircraft noise and cardiovascular effects have been carried out on adults and on children focussing on mean blood pressure, hypertension and ischemic heart diseases as cardiovascular endpoints. While there is evidence that road traffic noise increases the risk of ischemic heart disease, including myocardial infarction, there is less such evidence for such an association with aircraft noise. This is partly due to the fact that large scale clinical studies are missing. There is sufficient qualitative evidence, however, that aircraft noise increases the risk of hypertension in adults. Regarding aircraft noise and children's blood pressure the results are still inconsistent. The available literature was evaluated for the WHO working group on «DQ»Aircraft Noise and Health«DQ» based on the experts' comprehensive knowledge in this field. With respect to the needs of a quantitative risk assessment for burden of disease calculations an attempt was made to derive an exposure-response relationship based on a meta-analysis. This association must be viewed as preliminary due to limitations which are concerned with the pooling of studies due to methodological differences in the assessment of exposure and outcome between studies. More studies are needed to establish better estimates of the risk.



How to cite this article:
Babisch W, Kamp Iv. Exposure-response relationship of the association between aircraft noise and the risk of hypertension.Noise Health 2009;11:161-168


How to cite this URL:
Babisch W, Kamp Iv. Exposure-response relationship of the association between aircraft noise and the risk of hypertension. Noise Health [serial online] 2009 [cited 2020 Jul 4 ];11:161-168
Available from: http://www.noiseandhealth.org/text.asp?2009/11/44/161/53363


Full Text

 Introduction



The auditory system is continuously analyzing acoustic information, which is filtered and interpreted by different brain structures. The hypothesis that long-term exposure to environmental noise - including aircraft noise - causes adverse health effects is based on three major findings and facts:

Laboratory studies show that exposure to acute noise affects the sympathetic and endocrine system, resulting in unspecific physiological responses (e.g. heart rate, blood pressure, vasoconstriction, stress hormones, EEG). [1],[2],[3],[4],[5],[6],[7],[8]Noise-induced instantaneous autonomic responses do not only occur in waking hours but also in sleeping subjects even when no EEG awakening is present. [9],[10],[11],[12] They do not fully adapt on a long-term basis although a clear subjective habituation occurs after a few nights. [13],[14] Repeated arousal from sleep is associated with a sustained increase in daytime blood pressure. [15] The cortical perception of the sound as well as sub-cortical reflections due to the direct nervous interactions of the acoustic nerve with hypothalamic structures stimulates the autonomous nervous system. From this the hypothesis emerged that long-term exposure to noise adversely affects the homeostasis of the human organism, including metabolic function and the cardiovascular system. [16],[17],[18],[19],[20] Persistent changes in endogenous risk factors due to noise-induced dysregulation promote the development of chronic disorders such as atherosclerosis, hypertension and ischemic heart diseases and others in the long run.Although effects tend to be diluted in occupational studies due to the "healthy worker effect", epidemiological studies carried out in the occupational field have shown that employees working in high noise environments are at a higher risk for high blood pressure and myocardial infarction. [21],[22],[23],[24],[25],[26] Similar effects may occur with respect to community noise.

The general stress theory referring to the sympathetic-adrenal-medullar system (SAM axis) and the pituitary-adrenal-cortical system (hypothalamic-pituitary-adrenal = HPA axis) is the rationale for the non-auditory physiological effects of noise.[27],[28] The biological plausibility derives from laboratory experiments on acute noise effects. Epidemiological studies have been carried out assessing the relationship between road and aircraft noise on cardiovascular endpoints.

 Protocol of the Review



The focus here is on epidemiological studies or surveys directly related to associations between aircraft noise and cardiovascular disease (CVD) outcomes. Many environmental noise studies refer to road traffic noise, serving as an approximation of effects of transportation noise, in general. In accordance with the reaction model, the endpoints considered in this review are primarily of cardiovascular nature. Noise research has been focusing on these endpoints for reasons of statistical power (high prevalence in the general population) and their impact on public health. [29] A distinction is made between the effects on adults and on children. Clinical manifestations of cardiovascular diseases are not very likely in young people. Therefore blood pressure reading is the major outcome that has been studied in children and adolescents. In adults, however, manifestations of high blood pressure (hypertension) and ischemic heart diseases (myocardial infarction, angina pectoris, ischemic signs in the ECG, heart failure) are major outcomes of interest. The diagnosis is either based on self-reported doctor-diagnosed occurrence and/or treatment of disease, hospital admission rates, drug medication intake, or on actual blood pressure measurements (taken at rest). The same applies to the assessment of exposure. It is either based on self-reported traffic volume (e.g. type of street) or subjective perception of the noise (disturbance/annoyance), or on modeled noise contours (noise maps, isophones) or noise measurements taken near the subjects' houses. Finally, the type of study (ecologic, descriptive (e.g. cross-sectional study), and analytic (e.g. case-control study, cohort study) is considered as a decision criterion.

 Identification of Relevant Studies



The selection of relevant studies is made on comprehensive previous reviews [21],[30],[31] and the experts' knowledge about new publications and ongoing research in this field. In a recent review update [32],[33] altogether 61 epidemiological studies were identified that addressed the association between transportation noise and cardiovascular endpoint; 20 of which referred to commercial aircraft noise, [34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46],[47],[48],[49],[50],[51],[52],[53],[54],[55] 8 to military aircraft noise, 32 to road traffic noise, and 13 to other environmental noise sources. The cardiovascular chapters of the WHO reports "Night Noise Guidelines" [12] and "Environmental Noise Burden of Disease" refer to this review. [56] Studies focusing on low flying jet-fighter noise showed higher blood pressure readings in children but not in adults. [57],[58],[59],[60],[61] The effects may largely be due to anxiety and fear rather than to the noise stress as such. These studies are therefore not considered in this present summary on the effects of aircraft noise. However, studies regarding noise from aircraft operations around airfields, which is comparable to commercial aircraft noise (no steep level increases) are considered. [50],[62] New aircraft noise studies are now available that were not considered in previous reviews. [63],[64],[65],[66],[67],[68],[69]

Evaluation criteria for the validity of studies with respect to possible exposure misclassification, confounding, selection bias, recall and observation bias were: Objective (noise level) vs. subjective exposure assessment, objective (clinical) vs. subjective assessment of outcome, type of study, reasonable control of confounding factors, statistical methods of analyses, peer-reviewed reference.

 Studies on Adults



Some studies are not feasible for a synthesis or a meta-analysis, either because only sparse information is given with respect to the study design and selection criteria or confounding factors are insufficiently accounted for. [34],[53] Some study results are only preliminary or not yet peer-reviewed. [46],[54],[55],[68] However, in those cross-sectional studies - although mostly not significant - higher mean blood pressure readings or a higher prevalence of cardiovascular disorders or medication intake were found in exposed subjects compared with non-exposed, supporting the hypothesis as such (consistency). [70]

Repeated studies carried out around Schiphol airport in the Netherlands looking at aircraft noise and drug medication either on an individual level (self-reported medication intake) or on a spatial level (prescribed medication purchased by pharmacies) revealed higher relative risks of cardiovascular medication ranging between 1.2 and 1.4 for a noise level difference of approximately 10 dB(A). [37],[47],[64] When comparing the noise exposure throughout the whole day (L den ) with the noise exposure during the night (L night ) effects were stronger with respect to L den . In the most recent phase of the Schiphol environment and health monitoring programme a higher risk of approximately 1.8 was found for the same noise level difference. [65],[66] In a longitudinal approach a decrease in the purchase of cardiovascular and antihypertensive drugs was found after a reduction of night flights. [39] A recent cross-sectional study carried out around Cologne airport in Germany demonstrated higher individual prescriptions of antihypertensive and cardiac drugs in subjects exposed to high levels of aircraft noise, particularly, during the night and the early morning hours (3-5 hrs). [63] The study was unbiased with respect to the assessment of exposure and outcome because objective data were used (noise contours, health insurance records). However, no data regarding individual confounders were available, only spatially aggregated covariates could be considered. Higher risks were found for subjects where L night exceeded 39 dB(A). Preliminary results from a Swedish follow-up study carried out around Stockholm's airport suggest a higher intake of antihypertensive medication in subjects exposed to noise levels ('FBN') of more than 55 dB(A) compared to less exposed (relative risk 1.6). The results are based on a small sub-sample of the total cohort. [55]

Regarding the prevalence of hypertension and heart problems much information is derived from Dutch studies carried out around Schiphol airport. [37],[38],[65],[66],[71],[72] The assessment of high blood pressure and ischemic heart problems was based on clinical measurements, [37],[38] medical interviews, [37],[38] hospital admission rates, [65],[66] and self-reported hypertension. [65],[66] In the older studies, a non-significant increase in risk of heart disease was found ranging between 1.1 and 1.4 in people (males and females taken together) who were exposed to 'NNI' >37 (approximately L dn > 62 dB(A)). [37] For hypertension a significantly higher risk of 1.7 (95% CI = 1.4-2.2) was found for this noise level difference of approximately 10 dB(A). [37] Regarding the prevalence of all cardiovascular diseases, including high blood pressure, a significant relative risk of 1.8 was found. [38] In the later studies, no noise effects were found with respect to hospital admissions for cardiovascular diseases. [65],[66] However, a statistical significant effect of L den was found on self-reported hypertension. When the noise level increases by 3 dB(A) the odds ratio was 1.2, which corresponds with a relative risk of approximately 1.8 for a 10 dB(A) difference in noise level, confirming the earlier studies. In a new multi-centred study carried out around six European airports a significant increase in the risk of hypertension of 1.1 (95% CI = 1.0-1.3) for a 10 dB(A) difference of aircraft noise during the night (L night ) was found. [67] Hypertension was determined by a combination of three criteria: Measured resting blood pressure (systolic/diastolic blood pressure >140/90 mmHg), self-reported doctor-diagnosed hypertension, anti-hypertensive medication (ATC coding). Across categories no clear exposure-response relationship was found. However, the large confidence intervals did not discard the assumption of a linear relationship. No such association was found with respect to the exposure during the day, possibly due to exposure misclassification (time spent away from home). Thus, a smaller relative risk was found for the 24 hr noise indicator L den of 1.1 (95% CI = 0.9-1.3) per 20 dB(A). [Note: Because the data were previously not published by the Hyena group, the exact data are given here (OR per 10 dB(A) = 1.037, 95% CI = 0.962-1.119)].

A Swedish study carried out around Stockholm's major airport assessed the prevalence of (self-reported doctor-diagnosed) high blood pressure by postal questionnaire. An exposure-response association between aircraft noise and high blood pressure was found with relative risks ranging between 1.1 and 2.1 for noise levels between approximately 'FBN' = 53 to 63 dB(A). [52] When noise categories were combined, the effect was significant for 'FBN' > 55 dB(A). The trend analysis resulted in a relative risk of 1.3 (95% CI = 0.8-2.2) per 5 dB(A). Studies carried out around the Kadena military airfield on the Japanese island of Okinawa also demonstrated an exposure-response relationship of an increasing prevalence of clinically assessed hypertension with increasing noise exposure. [50],[73],[74] The effects were found at higher noise levels than for civil airports ('WECPNL' > 75 dB, approximately L dn > 60 dB(A). This may be due to the fact that night- and weekend-flights were largely omitted. However, older noise data were used which might not have adequately reflected the exposure when the health data were assessed. Only one prospective study assessing disease incidence is known. The study was carried out around Stockholm's major airport. The association between aircraft noise and high blood pressure was investigated. Subjects exposed to weighted energy-averaged levels ('FBN') above 50 dB(A) had a significant relative risk of 1.2 for the development of hypertension over the 10-year follow-up period compared with less exposed. [69] The increase in risk per 10 dB(A) was 1.2 (95% CI = 1.0-1.2). The effect was particularly found in older people, which may reflect longer years of residence.

 Studies on Children



Most evidence in relation to aircraft noise on children is derived from school studies carried out in Los Angeles, [40],[41] the Munich Airport study, [42],[43],[75] the Sydney Airport study, [44],[45] and the RANCH study. [76]

In studies around the Los Angeles airport blood pressure differences of 2 to 7 mmHg were found between groups of exposure depending on the years enrolled in school. The results may be confounded by incomplete control of ethnicity. [45] Blood pressure measures were taken during quiet periods in school, in order to exclude acute noise effects. Longitudinal measurements after a year failed to show a relationship between noise exposure at school and a change in blood pressure, probably due to selective migration of the schoolchildren. The cross-sectional study around the old Munich airport revealed a borderline significant effect of 2 mmHg higher systolic blood pressure readings in schoolchildren from noise exposed areas (L eq, 24hr = 68 dB(A)) as compared to unexposed children (L eq, 24hr = 59 dB(A)). No noise effect was found with regard to diastolic blood pressure. [42] Longitudinal studies carried out around the new airport showed a 2 to 4 mmHg larger increase in BP readings in exposed children than in their counterparts from the quiet areas 18 months after the opening of the new airport. However, the well-matched children from the exposed and the control group had the same absolute blood pressure. The higher change in blood pressure was due to lower values at the beginning of the follow-up. The cross-sectional study around Sydney Airport revealed a non-significant relation between aircraft noise and diastolic and systolic blood pressure in children. [45]

In a cross-sectional study carried out around Schiphol and Heathrow airports on schoolchildren (RANCH) a non-significant relationship was found between aircraft exposure at school (LAeq, 7-23 hr) and measured systolic blood pressure, diastolic blood pressure and heart rate after adjustment for relevant confounders. [76] However, aircraft noise at home (expressed as LAeq, 7-23hr) was significantly related to higher systolic (0.10 mmHg/dB(A)) and diastolic (0.19 mmHg/dB(A)) blood pressure. Chronic aircraft noise exposure during the night (LAeq, 23-7hr) at home was also positively associated with blood pressure. This latter association was significant only for systolic blood pressure. In the pooled data-set an increase of 0.09 mmHg/dB(A) was found. Due to significant differences in noise effects between the two centres no univocal conclusions about the association between aircraft noise exposure and blood pressure in children could be drawn. [76] Explanations put forward concern differences in flight pattern variation, and aircraft fleet. Also differences in schooling systems and teachers' attitudes towards noise might have differential effects on the children's reactions to noise. None of these could be tested on the available data. Finally, even though the results were adjusted for ethnic differences and diet residual confounding due to these factors might explain the differences. [77]

 Meta Analysis



Different approaches have been used to assess pooled effect estimates and exposure-response relationships in order to carry out a quantitative risk assessment. Van Kempen et al . [21] calculated uniform regression coefficients across all noise categories within individual studies ('regression approach'). The regression coefficients were then pooled over all studies. Babisch [32] calculated pooled relative risks for individual noise categories from different noise studies, which were then considered for an exposure-response relationship ('category approach'). Both approaches have advantages and disadvantages. The regression approach has the advantage that regression coefficients can easily be pooled regardless of actual noise levels; only the slopes (regression coefficient) of the exposure-response relationships of individual studies are taken into account, regardless of (different) noise level ranges and possible thresholds of effect. For example, some studies showed high risks at relatively low noise levels, [52] while others showed an increase of risk only at higher noise exposures. [50] The category approach is noise level oriented. Only relative risks from different studies referring to the same noise category are pooled to derive an exposure-response curve. This has the advantage that possible thresholds of effects can be determined. The approach also accounts for non-linear associations. It is less likely to obscure possible higher risks in higher noise categories where the numbers of subjects are often small - which is the case in random population samples given the empirical noise distributions, and specifically around large airports. For example, in case of j-shaped or quadratic associations an overall regression coefficient underestimates the risks in higher noise categories, simply because the slope of the regression line is primarily determined by the larger numbers of subjects in the lower exposure categories, where effects may be smaller. The disadvantage of this approach is that it relies on relatively homogeneous and comparable noise indicators in order to pool the effect estimates from different studies within noise categories. One could think of studies where relationships within the studies reflect true associations (slope), but the noise assessment in absolute terms may not be comparable due to methodological reasons (e.g. measurement vs. modeling, different calculation methods, different time periods, weighing factors, different reference points, different sides of the house, etc.).

For both approaches it is essential that critical decisions are made as to which studies are included in the meta-analyses and which are not. Studies that are not suitable with respect to issues of exposure misclassification, selection bias, observation bias, or confounding should be excluded from the meta-analyses. Only very few epidemiological studies are available on adults, in which the association between aircraft noise and clinical states of cardiovascular diseases were assessed. Five studies appear reasonably valid for further consideration because minimum requirements regarding the validity of the assessment of exposure, outcome and the statistical control for confounding factors were fulfilled. [37],[50],[51],[52],[67],[69] However, noise level related data pooling ('categorical approach') is difficult due to the fact that different (national) exposure indices were used. A graphical presentation of results using approximations with respect to the common noise indicator L dn is shown in [Figure 1]. No conclusions regarding possible threshold values or noise level related risks (in absolute terms) can be drawn.

When linear trend coefficients of all the five studies are calculated and pooled afterwards ('regression approach') the pooled effect estimate of the relative risk is 1.13 (95% CI = 1.00-1.28) per 10 dB(A). The results are shown in [Table 1]. The pooled effect estimate is significant. No major difference between fixed and random effect models is found when the individual coefficients obtained from the six airports of the HYENA study are considered individually in the meta-analysis to better account for the heterogeneity between individual studies. (Note: If the pooled Hyena results are used instead as shown in [Figure 1], significant fixed and random effect estimates of 1.12 and 1.29, respectively, are calculated.) The result is almost the same when either the 'Okinawa study' (military aircraft noise, out-dated noise data) or the 'Stockholm1 study' (subjective assessment of exposure) or both are excluded from the meta-analysis due to their low statistical weights (OR = 1.12, 95% CI = 0.98-1.28).

The calculations were made using the procedures 'Meta' and 'Metareg' of the statistical package STATA, Version 9. Individual odds ratios and confidence intervals were taken from summary reports [32] and the original publications for this purpose [67],[69] to calculate regression coefficients of individual studies and odds ratios with respect to the weighted day/night noise indicator L dn , which is supposed to be very similar to L den . [78] Approximations for the conversion of noise indices were given elsewhere. [79]

The noise assessment in the studies was made according to national regulations and calculation methods that were used by the time the studies were carried out in those countries. In the Amsterdam study [37] the Dutch 'Kosten Units' noise index (B) was calculated which considers the average maximum noise level of overflights during a 24-hour period and the number of events which are weighted by the time of the day (day 1.33, evening 5.25, night 9.75). The averaging is done on a sound-energy basis. The assignment of noise levels was based on maps of aircraft noise (1974) as modelled by the National Aerospace Laboratory. [80] In the Swedish studies (Stockholm 1 and Stockholm 2) [52],[69] the 24h-hour time weighted yearly equal energy level (FBN) was calculated; the number of events were weighted by the time of the day (evening 3, night 10). GIS-based noise dispersion models were used to define the noise contours. The Hyena consortium used the American INM (version 6.0) as uniform standard for all airports considered in the study to calculate yearly average noise contours for the day, the evening and the night (except the UK, where the national standard (Ancon model) was applied). The calculation was based on radar tracks of fight paths and the composition of aircrafts. The weighted 24-hour noise index L den according to the European Noise Directive was calculated (weighting: Evening + 5 dB(A), night + 10 dB(A)). In the Japanese study [50],[51] the noise assessment was based on continuous long-term and point selective short-term noise measurements based on monitoring programmes. The noise index WECPNL considers the average maximum noise level and the number of events. The events were weigthed by the time of the day (early evening 3, late evening and night 10). As pointed out earlier, the year of noise assessment did not coincident with the year of the health assessment, which raises some concern regarding exposure misclassification of which the direction of the impact on the results is unclear.

One also has to bear in mind that different criteria and assessment methods for hypertension were used. For example, some studies (Amsterdam, Stockholm1, Okinawa) refer to the 'old' WHO criterion of 160/100 mmHg, [37],[51],[52] others (Hyena, Stockholm 2) refer to the 'new' WHO criterion of 140/90 mmHg. [67],[69] It was assumed that relative (noise) effects were independent of the absolute prevalence of hypertension depending on the cut-off criterion for high blood pressure.

 Discussion



In the present summary, only those studies were considered in which aircraft noise was the explicit noise source. However, in a situation where information is lacking, the results of studies on the association between road traffic noise and myocardial infarction may also serve as an approximation for possible effects of aircraft noise. Considering the fact that at the same average noise level aircraft noise tends to be more annoying than road traffic noise, [81],[82] this approximation may even underestimate the effects of aircraft noise. Differences in the acoustical characteristics of the type of noise (e.g. frequency spectrum, quasi continuous road noise vs. single event aircraft noise, maximum noise level, length of single events, number of events), as well as non-acoustical factors (e.g. fear of aircraft crashes, attitude towards airport, effectiveness of coping strategies) may have an impact not only on the subjective perception of the noise, but also on physical health. Since aircraft noise comes from the top shielding of buildings is less effective. Because there is no access to a quiet side, sleep may be more affected by aircraft noise on a population level.

The available results do not allow for a distinction between the sexes. Males have been studied much more often than females. There is some indication that males may be more affected by road traffic noise. [67],[83],[84],[85] However, contradictory results were also found. [86] The data-base is too weak for final conclusions regarding any gender differences. Due to the use of different noise indicators in aircraft noise studies only very crude comparisons can be made between studies on the basis of common noise indicators, e.g. L dn or L Aeq,6-22hr . Most aircraft noise studies did not distinguish between day and the night. A road traffic noise study and two aircraft noise studies suggest that noise during the night may be more harmful than during the day. [63],[67],[87] However, no firm conclusions can be drawn about the relative contribution of day and night exposure because noise indices are usually highly correlated. One study suggests not only that noise during the night may be the primary source of adverse effects; it also shows that within the night period, effects due to noise in the early morning shoulder hours may be larger. [63]

The impact of noise on children's blood pressure is still not fully understood. Pre-dispositional and lifestyle factors seem to dominate and it is hard to study the influence of environmental noise separately. This might be one of the reasons why conclusions about the effect of noise exposure on children's blood pressure are limited and inconsistent. Methodological problems which arise are study size, insufficient contrast between noise levels, selection bias and insufficient adjustment for factors such as socioeconomic status, parental history, noise insulation and ethnicity. Moreover, most studies on cardiovascular effects in children have focused on school exposure while at least the combination of day- and night time exposure and the related lack of restoration might be of importance in the development of cardiovascular disease due to early childhood blood pressure changes.

Energy-based indicators of exposure (L eq ) are adequate and sufficient for the assessment of the relationship between long-term exposure to community noise and chronic diseases, e.g. cardiovascular disorders. These include L day,16h , L day,12h + L evening,4h , and L night,8h . Different periods of the day should be considered. Only if detailed data are not available L 24h is recommended. Although Leq -based indicators tend to be highly correlated in many exposure conditions, it remains unclear whether weighted indicators, such as L dn or L den reflect the physiological response of the human organism appropriately. However, when all information is available, weighted and non-weighted indicators can easily be calculated for use in health studies and related quantitative risk assessment.

 Conclusion



The general conclusion is that there is sufficient evidence for a positive relationship between aircraft noise and high blood pressure and the use of cardiovascular medication. Depending on whether high blood pressure was assessed by a self-administered postal questionnaire or by clinical measurements in studies, the magnitudes and the possible thresholds of effect varied between and within studies. [66],[68] Effects were more pronounced, when subjective measurements of high blood pressure were considered. This may raise questions regarding over-reporting. [66],[68],[88] The validity of study results appears to be even more a problem when subjective noise annoyance was considered for exposure. [47],[65],[66],[68] The effect estimates tend to be larger but may be prone to over-reporting, particularly in cross-sectional studies where both, exposure and outcome, are assessed on a self-reported basis with the same questionnaire.

No single, generalized and empirically supported exposure-response relationship can be established yet for the association between aircraft noise and cardiovascular risk due to methodological differences between studies (noise assessment, noise indicators, definition of hypertension) and the lack of continuous or semi-continuous (multi-categorical) noise data provided in the publications. For the same reason no answer can be given regarding possible effect thresholds. However, in spite of these limitations an attempt has been made to derive a "best guess" estimate, which can be used for practical purposes of quantitative risk assessment for the moment until more data are available. The calculated relative risk for an increase ("regression approach") of the day/night average weighted sound pressure level of aircraft noise of 10 dB(A) based on the presented meta-analysis is OR = 1.13, 95% CI = 1.00-1.28, range = 45-70 dB(A). Since this effect estimate is based on different slopes from different studies with different noise level ranges and methods being used, a decision must be made by the user with respect to the noise level onset of the increase in risk. Road traffic noise studies suggest that the cardiovascular risk increases when the outdoor noise level during the day exceeds 60-65 dB(A) and 50-55 dB(A) during the night, respectively. [89] As to whether this information can be applied to aircraft noise remains unclear. However, this may be a conservative approach, considering the results of annoyance studies showing that aircraft noise effects may even be stronger than those of road traffic noise. Annoyance studies showed that aircraft noise was more annoying than road traffic noise of the same average noise level, [81],[82] which might partly be explained by less exposure misclassification (no shielding of aircraft noise, no unexposed rooms). New aircraft noise studies suggest that the risk may increase at even lower night noise levels. It is therefore suggested to use L den ≤ 50 or L den ≤ 55 dB(A) as a reference category of the exposure-response relationship. The respective relative risks for subjects who live in areas where L den is between 55 to 60 dB(A) and between 60 to 65 dB(A) would then approximate to 1.13 and 1.20, or 1.06 and 1.13, respectively.

 Acknowledgement



The authors would like to thank Elise van Kempen and Danny Houthuijs from The National Institute For Public Health and Environmental Protection of The Netherlands for their helpful comments.

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