CO Levels of Concern in Government Recommendations 
and Regulations, in Voluntary Industry Standards, and
in Physiology, Public Health and Veterinary Practice   
Information compiled by Albert Donnay. 
Reposted on 2/17/2011 with permission from
http://coconundra.info/CO_Levels.php
All CO levels below are stated in parts per million (ppm) unless otherwise noted. 
World Health Organization (WHO) standards are published in mg/m3.  
For those who want to do the conversion, 1ppm = 1.145 mg/m3 and 1mg/m3 = 0.873 ppm.  
For those who don't, remember that any level in mg/m3 is about 10% less when expressed in ppm. 

Levels in white refer to average outdoor CO levels 
Levels in red refer to US EPA, US OSHA and WHO regulatory limits 
Levels in blue refer to recommended exposure limits of ACGIH, ASHRAE, and NIOSH 
Levels in yellow refer to US and European consumer CO alarm standards  
Levels in green refer to exhaled breath ranges 
Levels in white refer to guidelines of the Baltimore City Fire Dept, which are typical of most FDs
0-0.5   = level of CO in clean fresh outdoor air, such as far out at sea or in remote wilderness. 
0.1-1   = level of increases in average outdoor CO--within current outdoor ranges of 0-5 ppm-- that are associated in over 100 epidemiological studies with significantly increased risks of mortality and morbidity from many cardiovascular and respiratory disorders and, in growing fetuses, with low birth weight and birth defects--even after adjusting for the effects of other pollutants (ozone, SO2, NO2 and particulates) in multi-pollutant analyses. 
0-2                  = CO level in exhaled breath of healthy non-smokers who do not live with gas ovens, but only if not recently CO poisoned, not acutely stressed, and for women, not in the premenstrual phase of their cycle.  
0-3                  = range of max 8-hour avg. ambient CO in most US cities.  This range has declined significantly since 1970s when above 9ppm as use of catalytic converters in vehicle exhaust became more common.  See EPA graph of ambient CO data.   
0-29                = CO range in which consumer CO alarms are allowed to continuously display ZERO but not allowed to display the actual CO level, according to CO alarm standards developed by Underwriters Laboratories (UL2034) in collaboration with the US Consumer Product Safety Commission.  (The display and alarm specifications of UL2034 are matched in a Canadian standard, CSA 6.19-01)
3-15                = CO level in breath of non-smokers with flu, PMS/PMDD, chronic diseases, chronic low-level CO exposure (such as living with gas ovens) or recent more acute CO exposure.  
7 mg/m3        = maximum (max) 24-hour avg exposure established by WHO for Europe in 2010.  Although ostensibly meant to allow less total exposure than other WHO CO standards (see below), this standard actually allows more than twice as much total CO exposure as WHO's max avg 8-hour exposure standard.
9                      = max 8-hour avg outdoor ambient CO level allowed by US EPA, unchanged since first adopted in 1971.
9         = max indoor CO level recommended by the American Society of Heating, Refrigeration and Air Conditioning Engineers in a voluntary building industry standard (ASHRAE 62.2).
10 mg/m3      = max 8-hour avg CO level allowed by WHO and the European Commission. 
10-30 = CO level in exhaled breath of smokers within one to two hours after they last smoked. Exhaled CO in smokers remains chronically above 5ppm until days after they quit smoking.  
25                    = max 8-hour avg CO level allowed in occupational settings for an 8-hour workday over a 40-hour workweek by the American Conference of Governmental Industrial Hygienists.
 
25                    = CO level above which Baltimore fire department requires firefighters to put on self-contained breathing apparatus. 
30 mg/m3      = max 1-hour avg CO level allowed by WHO. 
30                                = CO level at and above which consumer CO alarms are allowed (but not required) by
UL2034 to continuously display the actual CO level.  
30-50              = lowest CO range in which consumer CO alarms are required by European Norm standards (EN50291) to sound, but only after this level  has been continuously sustained for 2 hours.
30-70 +/-3      = lowest CO range in consumer CO alarms are required by UL2034 to sound, but only after this level  has been continuously sustained  for over 30 days.  This is meant to (and effectively does) insure that consumer CO alarms never provide any warning at CO levels below 70ppm.   
30-999+                     = CO level exhaled by smokers while smoking and by non-smokers during acute high level CO poisoning.  In any high level CO environment, your exhaled breath CO level will gradually rise but remain lower than the ambient level because you always absorb some of the CO you inhale as long as the level in air is higher than the level in your blood.  As soon as you stop smoking or move to a lower CO environment, you start exhaling