Get this , There are several different standards depending on organization.
Fed EPA Standard for air quality 1979 Clean air act:
Max exposure levels @ 9ppm for eight hours of exposures and 35ppm for one hour of exposure!
50ppm for eight hours and 200ppm for no more than 15 minutes!
American National Standards Institute, The American Gas Association and the Canadian Gas Association have different standards for flue gas emissions from specific types of appliances.
(This is from: www.Jandnent.com)
I would go with the EPA and check www.GAMAnet.org for some more info , print them suckers out and hand it to the client/ seller for safety sake since all WILL die in the house if not dealt with.
Plus to me this is an indication of a faulty install or confined room to small to supply combustion air / dilution air for the appliances like the FAU and water heater Kitchen stove.:shock: