Epigenetic regulation of gene expression determines cell fate and influences fundamental processes such as development, differentiation, and proliferation. On the other hand, epigenetic genetic information is relatively easily changed by environmental changes and its abnormalities can cause diseases such as cancer. One of the molecular bases of epigenetic regulation of gene expression is the histone lysine acyl modifications. Some of these lysine acylations, at least in part, are induced by the addition of endogenous carboxylic acids such as fatty acids to the ε-amino group of lysine residues via chemical reactions. We unintentionally take in various exogenous carboxylic acids every day into our bodies through diet or other means. Some of these carboxylic acids can add to lysine residues of histones and function as epigenetic marks. Indeed, we found that several environmentally derived carboxylic acids add to lysine residues of histones and that some of them induce gene expression changes. In this presentation, we would like to introduce the effects of adduct formation by environment-derived carboxylic acids on epigenetic gene expression and discuss the hidden functions of environment-derived carboxylic acids and their effects on the organism.