First passage times as environmental safety indicators: carboxyhemoglobin from cigarette smoke.

The concentration of carbon monoxide in the blood of a cigarette smoker varies in response to the frequency and dose of CO delivered by the cigarettes he smokes and by the rate at which CO washes out of his blood. Moments of first passage times or exit times above a nominal threshold can be calculated using a stochastic differential equation that takes into account certain random variations in smoking intervals. CO doses, and washout rate. Almost any additional source of variation decreases in length of time a smoker may expect to smoke until a threshold value is exceeded. In particular the methodology proposed by Gori (1976) and Gori and Lynch (1978) for constant intervals, doses and rate may greatly overestimate the length of the "low-risk" interval for carbon monoxide concentration. One possible modification that may reduce CO hazards is to smoke less from each cigarette, even when this may be partially compensated by increased frequency of smoking.