Studies into a relationship between noise exposure at work and the risk of high blood pressure are inconsistent in their results. Reason for a group of Chinese researchers to conduct a case-control study in a car factory. In a relatively young population of male employees, they do find a connection with exposure to noise above 80 dB(A).
Noise exposure and blood pressure
High blood pressure increases the risk of cardiovascular disease and may also be influenced by work factors, such as exposure to noise. Noise exposure can lead to physiological stress, which manifests itself in an increase in heart rate and stress hormone concentrations (cortisol, adrenaline and noradrenaline) in the blood. This can have pathophysiological consequences in the medium to long term, as evidenced by animal and human studies. Exposure to noise increases the plasma concentrations of noradrenaline and angiotensin II, which in turn lead to abnormalities in the endothelium and stimulation of the renin-angiotensin system and to high blood pressure.
Previous research
A meta-analysis of 23 studies showed that noise exposure increases the risk of high blood pressure by a factor of 1,72 (Bolm-Audorff et al. 2020). Earlier studies also showed that workers exposed to high levels of noise had higher blood pressure than those exposed to low levels. But there are also studies showing that exposure to noise between 80 and 85 dB has no effect on blood pressure (Stokholm et al. 2013). The link between noise exposure and high blood pressure is therefore still unclear.
Chinese study in car factory
Recently (2022) a case-control study was published on the relationship between noise exposure at work and high blood pressure in a Chinese car factory. All participants in the study work on the production lines. Their tasks include operating machines, painting, welding, sanding, polishing, emission testing, etc. Noise is mainly caused by sanding, certain destruction tests and the use of cutting tools, for example. It concerns both continuous noise and impulse noise.
Research group
5537 employees are available for the study. Of them, 5443 undergo a medical examination. Employees of departments without noise exposure and newly hired employees are excluded. This also applies to people with cardiovascular disease, liver and kidney disease and tumours. In the end, the data of 4919 employees were analysed; all men.
High blood pressure group (cases)
Blood pressure was recorded at the start of the day and after participants had been sitting for 10 minutes. Measured on both arms; three times with an interval of 1-2 minutes. The average of three measurements is recorded. High blood pressure is defined as a systolic blood pressure (SBP) ≥140 mmHg and/or a diastolic blood pressure (DBP) of ≥90 mmHg when no blood pressure medication is used (n=494). Employees with a history of high blood pressure and the employees who were currently taking blood pressure medication are also considered to be employees with high blood pressure, even if their measured blood pressure was lower than the above values (n=15). These employees with high blood pressure (509) form the study group.
Control group (controls)
For each of the cases, two control persons of the same age were sought from the group without high blood pressure. The total group eventually has 1527 employees.
Comparison of the included and excluded subjects showed no significant differences in smokers, family history of high blood pressure, duration of noise exposure, noise level and cumulative noise exposure. However, there were significant differences in BMI, total cholesterol, triglyceride, high and low density lipoproteins and heart rate.
Noise
The noise in the company has been measured by qualified occupational hygienists according to Chinese guidelines. Both continuous noise and impulse noise were measured at each relevant workplace. This also calculates a cumulative noise exposure: 10 × log (10SPL/10 × years of noise exposure). SPL = sound pressure level [dB (A)] or the sound pressure. There is occupational noise exposure at a noise level of ≥80 dB (A) (Lex, 8 hours) or cumulative noise exposure ≥80 dB (A)-years. With continuous noise, the fluctuation in noise level is less than 3 dB(A) over the measurement period, and with impulse noise it concerns sudden and short noise of <0,5 sec, an interval >1 sec and a change of >40 dB(A) .
Results
Before and after adjustment for the confounding factors mentioned above, the Odds Ratios (ORs) and the associated 95% Confidence Intervals (95% CI) were calculated. For correction it turns out a noise level ≥80 dB (A) (Lex, 8 am) related to high blood pressure with an OR = 2,54 (95% CI 2,02 – 3,19). A cumulative exposure ≥80 dB(A) years is also associated with high blood pressure with an OR = 1,56 (95% CI 1,24 – 1,97). There appears to be a non-linear relationship between noise exposure and high blood pressure.
Discussion and conclusion
The researchers find a positive association between occupational noise exposure and high blood pressure in workers exposed to noise above 80 dB(A). This is in line with previous research, although the results are also inconclusive and often small or retrospective in design. Remarkably enough, the risk seems to decrease again at higher noise levels and the researchers wonder whether wearing hearing protection at higher noise levels or healthy worker effect may play a role in this.
The researchers also discuss the possible combination of exposure to noise and chemical substances. Both forms of exposure seem to have an effect on high blood pressure. They have tried to correct for this, but they believe that further research into the combination is necessary.
Also read: Cardiovascular disease due to noise?
Sources
Zhou, B., Lan, Y., Bi, Y., Li, C., Zhang, X., & Wu, X. (2022). Relationship Between Occupational Noise and Hypertension in Modern Enterprise Workers: A Case–Control Study. International Journal of Public Health, 67, 1604997.
Bolm-Audorff, U, Hegewald, J, Pretzsch, A, Freiberg, A, Nienhaus, A, and Seidler, A. Occupational Noise and Hypertension Risk: A Systematic Review and Meta-Analysis. Int J Environ Res Public Health (2020) 17(17):E6281. doi:10.3390/ijerph17176281
Stokholm, ZA, Bonde, JP, Christensen, KL, Hansen, AM, and Kolstad, HA. Occupational Noise Exposure and the Risk of Hypertension. E (2013) 24(1):135–42. doi:10.1097/EDE.0b013e31826b7f76