Individuals spend a large proportion of their time indoors (residence, factory, etc.) with some studies reporting up to 87% on average (Klepeis et al., 2001). The shift to an indoor lifestyle and the accompanying decreased outdoor exposure during the last decades has increased the prevalence of many allergies (Platts-Mills, 2015). For example, children growing up on farms developed fewer allergies due to exposure to germs (von Mutius and Vercelli, 2010). Moreover, people are more exposed to indoor air pollutants such as VOC (Lundberg, 1996) or (ultra)fine particulate matter (e.g., from traffic that can be found both outdoors and indoors, Christian et al., 2022). Poor indoor air quality can negatively affect physical and mental health and thus lead to poor quality of life, what is manifested in terms like idiopathic environmental intolerance (IEI), and multiple chemical sensitivity (MCS; Nordin, 2020; Viljoen and Thomas Neé Negrao, 2021), or just a slight, clinically not relevant increase of environmental sensitivity.

The evaluation of indoor air quality depends on different factors, for example sociodemographic and psychosocial (work) factors. Brauer and Mikkelsen (2010) found that psychosocial work factors were related to the perception of the indoor environment at individual level, but building characteristics were not associated with complaints about indoor environment. Workplace-level psychosocial risk factors could not explain this response heterogeneity whereas type of organization (e.g., office, hospital, school) explained some of the differences in perception. Cheung et al. (2022) showed for the work and private context that people with higher job and life satisfaction rated the indoor environmental quality as more satisfying than people with less job and life satisfaction. Furthermore, in the same study, Cheung et al. (2022) investigated the relationship between the Big Five personality traits (extraversion, agreeableness, conscientiousness, emotional stability, and openness to experience) and indoor environment. Agreeableness was associated with satisfaction with overall workspace environment, but otherwise the Big Five were not associated with evaluations of temperature, humidity, air movement, freshness of the air (stuffiness), or odors. Another study in the work context (Budie et al., 2019) included the Big Five in a path model to examine employee satisfaction with the work environment. Among these traits only agreeableness was positively associated with satisfaction with indoor climate. Individual factors like demographic variables also seem to influence the perception of indoor air environment: Women were found to be less satisfied with the thermal environment at times when ventilating and air conditioning is necessary (Choi et al., 2010); and Karjalainen (2007) found that women were less satisfied with indoor temperature, prefer warmer rooms and rated rooms as more uncomfortably hot or cold than men. In addition, subjects over 40 years rated thermal environment at times when ventilating and air conditioning is necessary as more satisfying than subjects under 40 years (Choi et al., 2010). In sum, individual factors seem to influence the perception of environment. However, up to now environmental sensitivity has not been researched as intensely as MCS or IEI. To approach the phenomenon of increased environmental sensitivity, we therefore first present findings on these clinically discussed phenomena.

MCS belongs to the broader definition of IEI, which additionally includes other environmental sensitivities such as electro (magnetic) sensitivity (Rossi and Pitidis, 2018). Present etiological explanations are not unanimous. On the one hand, there are biological theories, including toxicological or immunological theories that suggest an initially higher or longer exposure that is normally tolerated, as a cause (Bauer et al., 2008; Genius, 2010; Terr, 1987). According to consensus criteria (Bartha et al., 1999), symptoms are subsequently triggered and manifested by low doses and the appearance of symptoms depends on the presence of chemicals. Thus, MCS is defined as a chronic condition with reproducible recurring symptoms in response to different chemical substances in low doses (Bartha et al., 1999; Rossi and Pitidis, 2018). On the other hand, there are psychological theories, which assume a misattribution of symptoms to environmental stimuli leading to associative learning/conditioning (Siegel and Kreutzer, 1997). These theories “tend [s] to report the source of such disturbances to the psyche, as an endogenous self-induced cause and not as a consequence of excessive and abnormal reaction to an albeit reduced chemical exposure” (Rossi and Pitidis, 2018, p. 139). Other theories are based on a model in which both approaches are relevant. The development of MCS is seen as a multistage and very individual process with different factors: Exposure and vulnerability lead to a sensitive individual (Bauer et al., 2008). A learning approach is used by Van den Bergh et al. (2017) who proposed a biopsychosocial mechanism to explain IEI/MCS. The nocebo effect occurs when subjects associate an odor with symptoms by learning effect. When they perceive the harmless odor again, they expect symptoms independent und dissociated of the current exposure level. According to Van den Bergh et al. (2017) this nocebo/learning effect is stronger if subjects present a high negative affectivity (trait), that is, negative affectivity might act as a moderator. The authors reported several reasons for this relationship, for example, subjects with high negative affectivity are more attentive to the affective elements of a somatic experience and perceive sensory-perceptual elements as less intensive.

After developing environmental sensitivity, individuals exhibit symptoms for many years as shown in some long-term studies. One-, 5-, and 9-year follow up studies about IEI, MCS, and chemical intolerance confirmed their chronic condition, with some individuals reporting improvement in symptoms such as headache, pain, and fatigue, e.g., due to hospitalization, books, support groups (e.g.,Bailer et al., 2007; Black et al., 2000; Skovbjerg et al., 2015). In addition, it was found that negative affect such as anxiety or neuroticism increases the development and persistence of IEI (Bailer et al., 2004; Skovbjerg et al., 2015).

In general, there is significantly more data on people with independently observed/diagnosed MCS according to various criteria (e.g., from Cullen (1987) or Bartha et al. (1999)) than on people who self-report it in questionnaires. Prevalence of these “diagnosed” MCS cases increased from approximately 2.5%–12.8% during the last 10 years until 2018, whereas self-reported MCS increased from approximately 11.1%–25.9% (Driesen et al., 2020; Steinemann, 2018). It can be assumed that even more people show increased sensitivity to environmental stimuli below any clinical thresholds and that these numbers will continue to rise. Also, Van den Bergh et al. (2017) assumed that MCS is “the tip of an iceberg of highly prevalent self-reported chemical hypersensitivity among the general population” (p. 552). Almost all studies examining gender differences reported women to be more likely to have self-reported or observed MCS than men (e.g., Andersson et al., 2008; Berg et al., 2010; Hausteiner et al., 2005).

Besides physical symptoms such as headaches and fatigue, many affected people reported an increase in irritability, anxiety, depressed mood, as well as negative affectivity (Azuma et al., 2019; Bailer et al., 2007; Hausteiner et al., 2005; Papo et al., 2006). A few studies have determined that individuals with MCS showed poorer mental well-being than healthy control groups (Georgellis et al., 2003; Johnson and Colman, 2017). Johnson and Colman (2017) found this result only in men, whereas there was no difference in positive mental well-being between women with and without MCS. Georgellis et al. (2003) found no differences in this relation between men and women. However, these studies investigated only well-being in general and not current well-being in an actual exposure situation. Overall, studies about the relationship between environmental sensitivity and current well-being during exposure are lacking. Changes in current well-being during exposure could help to separate general psychological reactions from reactions to the actual environment.

Another factor that could be influenced by environmental sensitivity is the perception of air quality, but studies show contradictory results. Alobid et al. (2014) showed that individuals with MCS were less able to identify odors compared to a control group. Moreover, affected people reported “more odours as being intense and irritating and less fresh and pleasant” (Alobid et al., 2014, p. 3203). In other studies, individuals with MCS expressed stronger odor perceptions (van Thriel et al., 2008) or had unpleasant sensations in response to more odors (Ojima et al., 2002). In contrast, other studies found that affected individuals were just as capable of identifying odors and perceiving exposures (Georgellis et al., 2003; Ojima et al., 2002) as well as perceiving the intensity of odors as the control group (Andersson et al., 2014; Georgellis et al., 2003). In one study, no difference regarding reported unpleasantness of smell was found (Georgellis et al., 2003).

To sum up, few studies investigated the relationship between environmental sensitivity and perception of air quality and odor intensity under exposure. However, the results are contradictory. Furthermore, no studies explored the relationship between environmental sensitivity and current mental well-being under exposure. Studies that determined effects of environmental sensitivity in the normal population, that is, beyond clinical phenomena, in their everyday environment such as office buildings or aircraft are missing.

This study explores the effect of increased sensitivity to environmental stimuli on perception of air quality and well-being in different exposure conditions. For this purpose, people of a non-clinical population were exposed to different levels of CO2 and VOC. The following hypotheses were tested:

H1) Regardless of the level of exposure, individuals with higher environmental sensitivity show worse current well-being than individuals with lower environmental sensitivity.

H2) Environmental sensitivity moderates the relationship between exposure and current well-being: individuals with higher environmental sensitivity show worse well-being at higher exposures than individuals with lower environmental sensitivity.

H3) Regardless of the level of exposure, individuals with higher environmental sensitivity perceive air quality worse than individuals with lower environmental sensitivity.

H4) Environmental sensitivity moderates the relationship between exposure and air quality perception: individuals with higher environmental sensitivity respond more strongly to higher exposures than individuals with lower environmental sensitivity.

Since the described clinical phenomena showed different prevalence in men and women, differential effects are considered. In addition, the increased negative affect found in some studies could also be explanatory. Therefore, it is also considered as a predictor and the findings are contrasted with the results of the analyses with increased environmental sensitivity.