Toi there is documentation of the Chazon Ish’s instructions for brain surgery (Shu”t Ugnazim Chazon Ish vol. 1), which subsequently saved the patients life. If the patient would’ve thought like you he’d have been dead.
Regardless, Chazon Ish himself didn’t think smoking was healthy (letter to Steiper ibid).
He also didn’t believe he knew all medicine as GAW thinks. He’d ask doctors questions regarding being mechalel Shabbos or Yom Kipur for sick people (Maaseh ish).