In the USA the Environmental Protection Agency (EPA) have set a maximum level of CO in the flue at 400 PPM. In the home a concentration of no more than 9 PPM is recommended. While at work during an 8 hour period the level should not rise above a maximum level of 35 PPM i.e. if you are exposed to 35 PPM for 8 hours you should be in fresh air (zero PPM CO) for the remainder of the 24 hours.

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