To the Editor: Recently Namkug Kim, Ph.D., et al. reported that altitude appears to be a significant independent risk factor for suicide, hypothesizing that hypoxia mediates this relationship (1). Cigarette smoking leads to increased carboxyhemoglobinemia, reduced hemoglobin availability for oxygen, resulting in impaired oxygen release to tissues (2—4). Thus, like altitude, smoking is likely to decrease oxygen availability in the brain.

Current cigarette smoking has been shown to be dose-dependently associated with increased rates of suicide, an association not found in ex-smokers (5). Whether this association can withstand adjustment for psychiatric and addiction confounding factors has also been debated (6, 7). Several mechanisms have been proposed to explain this association (5—7): the toxic effects of nicotine, monoamine oxidase activity inhibition, the high prevalence of psychiatric comorbidity and consequent suicide risk, and smoking-induced serious physical illness with pain and disability resulting in negative mood response. Smoking could also be an inadequate self-medication for psychological symptoms, themselves causing suicide, and finally the association could be due to a third underlying factor associated with both smoking and suicide. To our knowledge, hypoxia has not yet been hypothesized as a possible mechanism for the association of current smoking and suicide. The suggestion by Kim et al. (1) that hypoxemia may have a role in inducing suicide can particularly be applied for the current smoking-suicide relationship and would merit further investigations.