The previous WHO guidelines were established for 15 minutes to protect against short-term peak exposures that might occur from, for example, an un- vented stove; for 1 hour to protect against excess exposure from, for example, faulty appliances; and for 8 hours (which is relevant to occupational exposures and has been used as an averaging time for ambient exposures). We do not rec- ommend changing the existing guidelines. However, chronic carbon monoxide exposure appears different from acute exposure in several important respects. The latest studies available in 2009, es- pecially those epidemiological studies using very large databases and thus pro- ducing extremely high-resolution.
