His hearing began to fail in his mid to late twenties. He first described the problem in a letter of June 1801, at age 30, and was fully deaf by his mid forties.
Lead exposure. A 2024 letter in Clinical Chemistry tested two authenticated locks of his hair and estimated his blood lead at 69 to 71 µg/dL. Such levels are commonly associated with stomach and kidney problems and decreased hearing.
Patronage. In 1809, after he was offered a post as music director to Napoleon's brother in Kassel, Archduke Rudolph and Princes Kinsky and Lobkowitz agreed to pay him 4,000 florins a year. The only condition was that he decline the Kassel offer and stay in Vienna. His friend Ignaz von Gleichenstein led the negotiations.
The locks date from about 1820 to 1827, so they cannot show whether lead exposure preceded his hearing loss. The authors do not think lead alone caused his deafness or death. Lead exposure is well supported. Lead as the cause of his deafness is rated C.
Also relevant: Bell, A., Chetty, R., Jaravel, X., Petkova, N., & Van Reenen, J. (2019). Who becomes an inventor in America? Quarterly Journal of Economics, 134(2), 647-713. All references