Wasn't there something about some seed oil fumes being toxic? Maybe it's related to some cooking practices. Highly speculative but the cooking oil toxicity is well-known.
[edit 2] and indeed the article mentions it, although just in passing, still my hunch would go towards this as it seems a more specific factor than the others that are mentioned
[edit] found this with a quick search, seems relevant: Exposure to Cooking Oil Fumes and Oxidative Damages: A Longitudinal Study in Chinese Military Cooks https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4029104/
[edit 2] and indeed the article mentions it, although just in passing, still my hunch would go towards this as it seems a more specific factor than the others that are mentioned