In Sweden, as in many other European countries, traffic noise is an important environmental health issue. At present, almost two million people are exposed to average noise levels exceeding the outdoor national guideline value (55 dB(A)). Despite efforts to reduce the noise burden, noise-related health effects, such as annoyance and sleep disturbances, are increasing. The scientific interest regarding more serious health effects related to the cardiovascular system is growing, and several experimental and epidemiological studies have been performed or are ongoing. Most of the studies on cardiovascular outcomes have been related to noise from road or aircraft traffic. Few studies have included railway noise. The outcomes under study include morning saliva cortisol, treatment for hypertension, self-reported hypertension, and myocardial infarction. The Swedish studies on road traffic noise support the hypothesis of an association between long-term noise exposure and cardiovascular disease. However, the magnitude of effect varies between the studies and has been shown to depend on factors such as sex, number of years at residence, and noise annoyance. Two national studies have been performed on the cardiovascular effects of aircraft noise exposure. The first one, a cross-sectional study assessing self-reported hypertension, has shown a 30% risk increase per 5 dB(A) noise increase. The second one, which to our knowledge is the first longitudinal study assessing the cumulative incidence of hypertension, found a relative risk (RR) of 1.10 (95% CI 1.01 - 1.19) per 5 dB(A) noise increase. No associations have been found between railway noise and cardiovascular diseases. The findings regarding noise-related health effects and their economic consequences should be taken into account in future noise abatement policies and community planning.
Keywords: Cardiovascular disease, community noise, epidemiology, hypertension, myocardial infarction, noise abatement
|How to cite this article:|
Bluhm G, Eriksson C. Cardiovascular effects of environmental noise: Research in Sweden. Noise Health 2011;13:212-6
| Introduction|| |
Traffic noise is an increasing problem in modern society and it is the dominating source of noise in the urban environment.  In Sweden, almost two million people are exposed to average noise levels exceeding 55 dB (L Aeq,24h), which is the present national guideline value for equivalent outdoor noise levels at the façade. Almost one million (12%) are annoyed by noise outside or near their homes, once a week or more.  In the Swedish national environmental targets set in 1999, a detailed aim for noise reduction was settled. It was postulated that the number of persons exposed to community noise levels exceeding the guideline values of 55 dB L Aeq,24h and / or 70 dB L Amax should be reduced by 5% between 1998 and 2010.  However, the exposure to noise has instead increased. Noise-related health problems such as annoyance and sleep disturbances are also growing and more serious effects related to the cardiovascular system have been suggested. The scientific interest with regard to noise and health is increasing continuously in Sweden and several studies about the association between transportation noise and cardiovascular disease have been performed or are ongoing.
| Experimental Studies|| |
Few Swedish studies have investigated the cardiovascular effects of community noise exposure in experimental settings. One proposed biological mechanism for the possible causative relation between noise and cardiovascular diseases is that noise causes the release of stress hormones, which in turn affect the cardiovascular risk factor pattern. , Glucocorticoid hormone cortisol is the main secretory product of the neuroendocrine cascade and a good indicator of stress. During recent years, there has been an increasing interest in measuring saliva cortisol, which reliably reflects free cortisol levels in the blood. Saliva cortisol measurements also have the advantage of being carried out easily.  In the Hypertension and Exposure to Noise near Airports (HYENA) project, which is a multicenter study aiming at assessing cardiovascular health effects related to noise exposure from aircraft and road traffic, one aim was to study saliva cortisol as a possible marker for noise-induced stress.  In the study of saliva cortisol, which included a subsample of 439 subjects from the total HYENA population, a significant elevation in morning saliva cortisol level was observed in women exposed to aircraft noise levels above 60 dB (L Aeq,24h), in comparison to women exposed to noise levels lower than 50 dB (L Aeq,24h).  The Swedish part of this saliva cortisol study comprised of 85 study participants, who were selected to ensure a satisfying contrast in aircraft noise exposure. Among women, there was an increase of 1.09 nmol/l (95% CI - 0.12, 2.31) in the morning saliva cortisol level per 5 dB(A) increase in noise exposure. No association between noise exposure and saliva cortisol levels was found in men.
| Epidemiological Studies|| |
Several studies have been performed in Sweden on the associations between road and aircraft traffic noise and annoyance as well as the cardiovascular outcomes. Railway noise, on the other hand, has been less studied and has also been shown to be less disturbing than road and aircraft noise. However, an increasing number of complaints due to railway noise have recently been noted. This may in part be explained by a major expansion of railway traffic from Stockholm in the northward direction. At present, 450 trains are running along this line every day and the speed limit has been raised to 200 km/hour. In a regional questionnaire survey, performed in the Stockholm County, the prevalence of annoyance, sleep disturbances, and hypertension was assessed among people living along this busy railway line.  There was a clear association between self-reported annoyance and sleep disturbances and the estimated noise exposure levels. No differences in annoyance due to age or gender were reported. Regarding hypertension, there was no indication of an increased prevalence related to railway noise exposure, however, the duration of exposure was only two years at the time of the investigation. A study from the west of Sweden has also assessed the association between railway noise exposure and hypertension (self-reported and treatment), but has not found any increased risks. 
Road traffic noise
The association between road traffic noise and hypertension was studied in the HYENA project.  A significant relationship between average road traffic noise exposure and risk of hypertension was found in men, but not in women. In the Swedish part of this investigation, a country-specific odds ratio (OR) of 1.3 (95% CI 1.0, 1.7) per 10 dB increase in road traffic noise exposure (L Aeq 24h ) was found for hypertension.
Regional studies on road traffic noise in relation to annoyance and hypertension have been performed in Sollentuna, an urban municipality in Stockholm County. , The results for hypertension are shown in [Table 1]. A postal questionnaire provided information on individual characteristics including reported annoyance and diagnosis of hypertension. The odds ratio (OR) for hypertension, adjusted for age, smoking, occupation, and house type, was 1.38 (95% CI 1.06, 1.80) per 5 dB(A) increase in noise exposure. The association appeared stronger among women and among those who had lived at their address for more than 10 years. There was a clear exposure-response relation for hypertension as well as for annoyance.
|Table 1: Review of three Swedish epidemiological studies of high blood pressure in relation to road traffic noise published in 2006 - 2009 with adjusted risk ratios, with risk groups included|
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Two other Swedish cross-sectional studies have also indicated an association between road traffic noise and hypertension [Table 1]. The first study, from the southern part of Sweden,  was based on a large questionnaire survey and showed associations between treatment for hypertension and road traffic noise, only in subgroups. The analyses comprising of all study subjects indicated a positive dose-response relationship among women only. Among subjects reporting annoyance due to noise (males and females), a significant RR of 1.7 (95% CI 1.0, 2.7) was found for those exposed to 55 dB(A) or higher in comparison to those exposed to noise below this level. However, an exposure response relationship (three noise categories) was obvious only among the annoyed males and not entirely consistent among annoyed females. In the second study, from the west of Sweden,  a statistically significant association was found between a road traffic noise exposure above 55 dB(A) and self-reported treatment for hypertension. Allowing for >10 years of latency, the OR was 1.9 (95% CI 1.1, 3.5) when adjusting for age, sex, heredity, and body mass index (BMI). However, when stratifying for sex, a clear association was found for men only (OR 3.8, 95% CI 1.6 - 9.0).
The relationship between traffic noise and acute myocardial infarction has been sparsely investigated. Since 2003, only one study from Sweden has been published.  In this study, the objective was to study long-term traffic noise exposure in relation to first time myocardial infarction. The study was of case-control design and was based on the Stockholm Heart Epidemiology Program (SHEEP), which has been described in detail elsewhere.  The adjusted OR for myocardial infarction, comparing long-term exposure to road traffic noise levels above and below 50 dB (L Aeq,24h ), was 1.12 (95% CI 0.95, 1.33). In a subgroup, excluding subjects exposed to noise from other sources, the corresponding OR was 1.38 (95% CI 1.11, 1.71). It was notable that the point estimate did not change after adjustment for exposure to air pollution (NO 2).
The Swedish findings lend some support to the hypothesis of an association between road traffic noise and cardiovascular outcomes. Many potentially confounding factors, including smoking and occupational status were evaluated, but residual confounding could still be present, for example from noise at other locations. Disease outcome, which in some cases was based on self-reported data, might be a source of bias. However, it was reported that self-administered questionnaires could have good accuracy to confirm the outcomes, such as, hypertension.  As the exposure was assessed objectively by validated calculation models (through Geographic Information Techniques or manually) and was not dependent on the disease status, differential misclassification of exposure or disease was unlikely. In addition, the studies generally had a high response rate, which reduced the possibility of the results being influenced by chance or selection bias.
Aircraft noise has been linked to various physiological and psychological effects. Associations between aircraft noise exposure and ischemic heart diseases, such as myocardial infarction, angina pectoris, ischemic signs in the ECG, and heart failure, have so far not been established. However, recent studies have strengthened the evidence of an association between aircraft noise and hypertension. In the HYENA study, a significantly increased risk for hypertension was found for night time aircraft noise, per 10 dB(A) increase (L night). In the country-specific analyses for Sweden, the OR was 1.15 (95% CI 0.92, 1.42). 
Furthermore, two Swedish studies performed around Stockholm Arlanda airport indicate an increased risk for hypertension in the relationship to aircraft noise exposure. The results are shown in [Table 2]. The first study was of a cross-sectional design and included two random samples of subjects aged between 19 and 80 years.  The prevalence odds ratio for self-reported hypertension, adjusted for age, sex, smoking, and education, was 1.6 (95% CI 1.0, 2.5.) among those with energy-averaged aircraft noise levels exceeding 55 dB(A) FBN [FBN is the Swedish standard measure for aircraft noise exposure, which can be approximated to the day-, evening-, night-equivalent sound pressure level, L den , advocated in the European noise directive (EC, 2002)], and 1.8 (95% CI 1.1, 2.8) among those with maximum aircraft noise levels exceeding 72 dB(A). A trend analysis showed a risk increase of 30% per 5 dB(A) increase of noise. The other study around Stockholm Arlanda airport indicated an association between aircraft noise exposure and the cumulative incidence of hypertension in middle-aged Swedish men.  The study was based on two epidemiological surveys conducted 10 years apart, within the framework of the Stockholm Diabetes Preventive Program. The original study design is described in detail elsewhere.  This study is, to our knowledge, the first investigation of cumulative incidence of hypertension in relation to aircraft noise exposure. Among subjects exposed to energy-averaged levels above 50 dB(A) FBN, the adjusted relative risk for hypertension was 1.19 (95% CI 1.03, 1.37) compared to subjects exposed to noise below this level. For maximum aircraft noise levels, similar results were obtained with a relative risk of 1.20 (95% CI 1.03, 1.40) for those exposed to above, versus below, 70 dB(A). Additional results from analyses of the incidence of hypertension in relation to aircraft noise exposure around Arlanda, using a refined noise prediction model, were recently published.  This study investigated potential gender differences and found increased risks primarily among men.
|Table 2: Review of two Swedish epidemiological studies of high blood pressure in relation to aircraft noise published in 2001 - 2007 with adjusted risk ratios and risk groups included|
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The findings on aircraft noise exposure and hypertension are consistent with other recent international reports. The strengths of the Swedish studies include the objective measurements of both exposure and, in one of the studies, outcome (i.e. blood pressure measurements), which give high confirmation to the diagnosis. The prospective design and use of cumulative incidence data regarding hypertension in the latter study also make the results more valid. According to the findings from HYENA and other studies, annoyance due to aircraft noise seems to have increased during recent years, which is somewhat puzzling (Babisch et al., 2009).  In some studies, higher risks of hypertension are observed among subjects reporting annoyance due to aircraft noise. Thus, annoyance might be an effect modifier, indicating that annoyed subjects may be especially vulnerable to noise exposure. However, it may also be that these findings reflect less misclassification of exposure in this group.
In summary, recent Swedish studies support the hypothesis of an association between exposure to road and aircraft traffic noise and development of hypertension. An increased risk of myocardial infarction following long-term exposure to road traffic noise has also been indicated. However, the evidence is limited and additional high-quality longitudinal studies are needed. Regarding railway noise, a relationship with hypertension has hitherto not been observed but the number of studies is limited.
| Public Health Issues|| |
From a public heath point of view, the general awareness of community noise has been raised in Sweden during recent years. Several local action groups have been established, mainly claiming disturbances due to aircraft noise. The National Swedish Environmental Health report from 2009 showed a 40% increase in the number of noise annoyed subjects since 2001.  On a national level, the increase in annoyance was primarily related to road traffic (from 9 to 12%), however, in urban settings, such as in the Stockholm area, the increase in annoyance was seen for all noise sources [Figure 1].  There are several competing interests in the Swedish community planning. In big cities there is a housing shortage and extended urbanization is therefore supported by Swedish politicians. On the other hand, more people report annoyance and sleep disturbances due to noise exposure and serious cardiovascular health effects cannot be ruled out. The duration of exposure and bedroom positioning has been shown to be of special importance for the risk of developing cardiovascular disease, in Swedish studies. In order to avoid serious public health problems in the future, noise reducing strategies such as insulation of windows and assurance of access to a quiet side, need to be further implemented.
|Figure 1: Noise pollution and change over time. The percentage of population in the County of Stockholm, who report that they are disturbed by traffic noise (road, railway or aircraft noise) at least once a week|
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As a basis for risk assessment, a report has recently been published describing exposure to road traffic noise in the Swedish population (WSP Akustik 2009).  It was shown that during 2006, a total of 1.73 million persons (19%) were exposed to noise levels exceeding 55 dB(A) outside their residential setting, compared to 1.34 million (15%) six years earlier. There were two main reasons postulated for this difference: Since the year 2000, there has been an increase in traffic flow (10% in private cars and 30% in heavy traffic) and also in population growth (3%). The magnitude of the increase in noise exposure was, as expected, the highest in the largest cities. In Stockholm, Gothenburg, and Malmö, including the suburbs, there was a 50% increase in the number of subjects exposed to road traffic noise levels above 55 dB(A) between 2000 and 2006; from 530.000 to 790.000. Regarding railway traffic, about 225.000 subjects were exposed to noise levels exceeding 55 dB(A), in 2006, and for flight traffic, the corresponding number was 13.000. Health economic calculations predicted the costs related to noise exposure being about 2.2 billion Swedish crowns (≈230 million €) for the year 2006.  Although these figures are highly uncertain, the forecast conveys an important message when considering public health aspects in future urban planning, which can also serve as essential bases for further health economic analyses.
| Conclusions|| |
During recent years, there has been a growing interest and awakened public awareness regarding noise in relation to health, in Sweden. Several research studies have focused on noise-induced health effects, especially cardiovascular outcomes. Recent Swedish noise studies support the hypothesis of an association between residential exposure to road as well as aircraft traffic noise and hypertension. Increased risk has also been indicated for acute myocardial infarction in relation to road traffic noise. However, additional prospective research is needed. The findings regarding noise-related health effects and their economic consequences must be taken into account in future noise abatement policies and community planning.
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Institute of Environmental Medicine, Karolinska Institutet, Nobels väg 13, 171 77 Stockholm
[Table 1], [Table 2]