Abstinence from smoking and expired-air carbon monoxide levels: lactose intolerance as a possible source of error.

We investigated a possible source of contamination of expired-air carbon monoxide (CO) readings on CO monitors from raised expired-air hydrogen (H2) levels, namely lactose intolerance, the norm in certain ethnic groups. A significant correlation between expired-air CO and H2 readings was observed in four non-smoking lactose maldigesters.