AbstractAllergic diseases are an increasing global burden. Epidemiological and in vivo studies showed that farming environments could protect from allergic asthma. Studies explaining this protective effect mainly focused on the influence of chemical compounds in the molecular size range of proteins and endotoxins. Our study aimed at deciphering the possible role of small-sized semi-volatile organic compounds (SVOCs) of farming aerosols in immunomodulation processes. Bronchial epithelial BEAS-2B cells were exposed to aerosol extracts of particulate matter (PM_2.5) from farming environments. These cell exposures revealed a decisive effect of the smaller sized fraction (< 3 kDa) compared to extracts including the larger sized fraction. We demonstrated that smaller compounds can induce regulations of inflammatory and allergy-related genes including interleukin-8, xanthine dehydrogenase and toll-like receptor 2 (TLR2). Additionally, we performed a comprehensive chemical investigation of two typical farming aerosols (cow vs. sheep) by applying comprehensive gas chromatography coupled to time-of-flight mass spectrometry. We were able to identify several SVOCs characteristic for the protective cow sheds environment including four key components. Cell exposure with the two farming extracts showed a distinct regulation of the E3 ubiquitin ligase PELI2 gene and TLR2 by cow shed extracts. Finally, the regulation of TLR2 corresponded to the regulation that was observed after exposing cells to an artificial mixture of the four key components identified in the cow sheds. In summary, we were able to demonstrate the importance of smaller particle-bound SVOCs found in farming environments concerning their possible contribution to a protective farm effect.