According to the model of reaction to noise, noise produces a stress response. Specifically, noise activates the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system. When these systems are activated, stress hormones are released in the body, such as cortisol or adrenaline. The body has several mechanisms to return to its initial state from this stress response, but if the presence of the stressor is prolonged and becomes chronic over time, the body loses its ability to recover. This phenomenon is known as allostatic overload. It has been shown that this chronic stress causes chronic inflammation, damages the immune and metabolic systems, affects the intestinal microbiome and the regulation of the day-night cycle, and can cause cardiovascular problems.

To see how environmental noise can affect stress, its effect on cortisol levels has been studied, measured in various biological samples. On the one hand, cortisol levels measured in blood, saliva and urine samples are indicators of acute stress. On the other hand, in terms of chronic stress, the trend of using hair cortisol levels as an indicator of chronic stress and hypothalamic-pituitary-adrenal axis activity has increased significantly in recent years.

Regarding acute stress, mixed results have been obtained: some studies have concluded that participants living in high noise levels had higher cortisol levels. However, other studies have not obtained similar results. Regarding chronic stress, two studies have examined these variables before this study: in one, no relationship was found in 14-15 year old adolescents; and in the other, carried out with children from INMA-Gipuzkoa, higher exposure to environmental noise was associated with lower cortisol levels.

Consequently, driven by the lack of clear results, the aim of this study was to examine the effect of environmental noise on children's cortisol levels. For this purpose, data from the European project ATHLETE (Advancing Tools for Human Early Lifecourse Exposome Research and Translation) were used. This project arose from a concern: the lack of tools to develop knowledge about vulnerability to environmental factors in the early years of life and the exposure to these factors. Thus, this project was created with eighteen cohorts that were already formed, including the INMA cohort, which this research group has been working on for many years.

Of these 18 cohorts, 8 cohorts had cortisol information. In six of them, those included in the HELIX project, namely the BiB (United Kingdom), EDEN (France), KANK (Lithuania), MoBa (Norway), Rhea (Greece) and INMA-Sabadell cohorts, urine cortisol levels were available; and in the Generation R (Netherlands) and the Gipuzkoa and Sabadell cohorts of the INMA project, hair cortisol levels were available. As mentioned, urinary cortisol is an indicator of acute stress, while hair cortisol is an indicator of chronic stress. Therefore, the analyses were carried out in parallel with both data sets.

The main result was that there was no significant relationship between environmental noise and hair cortisol levels. The results were also similar for cortisol measured in urine: no relationship was observed between environmental noise and cortisol levels.

The research team explains that the results obtained do not necessarily indicate that there is no relationship between the variables studied. There may be several reasons for this result: On the one hand, an imprecise estimate of environmental noise could explain this result. In fact, environmental noise was estimated on the facade of the participants' homes, and this noise level may not necessarily be an indicator of the child's true exposure to noise, since exposure in other contexts was not taken into account. In addition, the individual's perception of noise is an important factor in studying the effect of environmental noise exposure on cortisol levels.

To fully understand the impact of environmental noise on health, it is important to study how it can affect biomarkers of chronic stress. The research team suggests that cortisol measured in hair is a useful tool for investigating the relationship between environmental stressors and accumulated cortisol levels. However, it is also important for future studies to include additional biomarkers of stress and chronic inflammation, such as other stress hormones or biomarkers of the immune system. From this point of view, the total allostatic load score is interesting. This indicator includes neuroendocrine, immune, metabolic and cardiovascular biomarkers. Using a comprehensive indicator of chronic stress could provide valuable information.