**3. Environmental factors contributing to asthma**

Parallel to genetic factors, environmental factors are also involved in the development and progression of asthma (**Figure 1**). The exposure to some environmental factors was shown to contribute not only to asthma but also to other related respiratory disorders, for example, emphysema development. By contrast, there are also some other environmental factors that seem to be solely linked to the development of asthma but not to other inflammatory or/and respiratory disorders [30]. Various studies assessed the risk factors of asthma and found evidence that allergen exposure, respiratory tract infections, gastroesophageal reflux disease (GERD), and physical and psychosocial stress might represent individual risk factors. It is important to keep in mind that some other environmental factors are protective, such as maternal diet, breastfeeding, and farming conditions [31].

Allergen exposure is the major factor impacting sensitization and constitutes the most common cause of asthmatic exacerbations in adults and children. A wide variety of inhaled allergens may trigger asthma symptoms, for example, house dust mite [32], pollens [33], cockroaches [34], and animal fur [35]. Respiratory tract infections have been implicated in asthma occur‐ rence and exacerbation as well. Examples include infection with viruses [36,37], *Mycoplasma* [38], and *Chlamydia species* [39]. Based on the conclusions from the Japanese study, which included 3085 patients, the change in weather followed by smoking was identified as two leading asthma-exacerbating factors [40]. Although (passive) smoking is a predominant contributing factor for the development of asthma [41], one occupational study [42] has shown that nonsmokers might also develop asthma due to occupational air pollutant exposure.

**Figure 1.** Environmental stress, in conjunction with genetic factors, both contribute to the development of asthma exac‐ erbations.

Additionally, a correlation has been observed between the presence of asthma and gastroeso‐ phageal reflux-induced disease, with reports showing one-third of asthmatic patients also diagnosed with GERD [43,44]. Although the coexistence of GERD in asthmatic patients did not affect asthma severity, the airway resistance was significantly higher in asthmatic patients with GERD [45]. Some other psychosocial factors such as parental stress during childhood [46] and the socioeconomic status [47] are reported to influence allergic inflammation severity. It is estimated that psychopathology is six times more common among asthmatics, and accord‐ ingly it correlates more closely with the asthmatic quality of life, rather than with lung physiological functions [48,49]. In both directions, psychopathology is supposed to precipitate asthma or vice versa; psychopathology may develop as a consequence of asthma [50].
