The term stress was first defined by Selye in 1956 as an adaptive reaction of an organism against the demands from its medium [1]. Stress is an adaptive response of an organism against a physical or psychological demand. The self-perception of stress appears when a subject cannot meet these demands [2]. A stressful situation initiates a neurohormonal activation system which include the increase of biochemical markers. One of the most-utilized biomarkers used to quantify stress is the activity of salivary alpha amylase (sAA) [3]. This is one of the main salivary enzymes that is secreted as a response to stimuli from the sympathetic nervous system [4] (SNS), so it is considered as the biomarker of choice in situations of acute psychologically-stressful stimuli [5].

Stress is a considerable problem among health workers, especially for those who perform their activity in emergencies, as they very often have to face complex situations that lead to stress [6]. It has been demonstrated that stress affects the performance of health professionals [7]. The unpredictable situations they face require the fast making of decisions, and stress can lead to the inadequate management of the threats against the life of a patient [8]. In the last few years, health sciences training programs have included the use of methodologies based on the participation in high-fidelity clinical situation scenarios [9]. There is a relationship between the stress generated through clinical simulation in emergency-based scenarios and the influence on the performance in an academic setting [10]. However, research is lacking on the levels of stress of these healthcare professionals in real situations of out-of-hospital emergencies handled by an Emergency Medical Service (EMS).

This study aims to shed light on the prevalence and severity of stress among EMS professionals worldwide, acknowledging the universality of stress in high-pressure environments and its critical impact on effectiveness and safety in patient care. By understanding these factors in a specific context, we seek to contribute to the international body of knowledge on stress management in emergency situations. The main objective of this work was to determine the increase in stress experienced by EMS professionals when handling real emergencies. The secondary objectives were to determine if there were differences in the increases in stress according to sex, age, professional category, and overall, as a function of the type of emergency handled.