Carbon Monoxide Conundra
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CO Levels of Concern in Government Recommendations
and Regulations, in Voluntary Industry Standards, and
in Physiology, Public Health and Veterinary Practice   

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 t
o 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 more CO than you inhale, but people who make more CO endogenously due to stress, disease, pregnancy or PMS may continuously exhale 3 to 15 ppm even without any recent exposure to exogenous CO sources. 

35 max 8-hour avg. workplace ambient CO level recommended by US NIOSH. 

35 = max 1-hour avg. outdoor ambient CO level allowed by US EPA, unchanged since first adopted in 1971.   

50 = max 8-hour avg workplace ambient CO level allowed by US OSHA. 

50-100 = lowest CO range in which consumer CO alarms are required by EN50291 to sound but only if and when this level has been continuously sustained for 1 to 1.5 hours (compare with UK smoke alarm standards that require instant warning as soon as smoke is detected).   

60mg/m3 = max 30-minute ave CO level allowed by WHO.

70-150 +/-5 = CO range in which consumer CO alarms are required by UL2034 to sound but only if and when this level is continuously sustained  for 1 to 4 hours (compare with UL & CSA smoke alarm standards that require instant warning as soon as smoke is detected).

100 mg/m3 =  max 15-minute avg CO level allowed by WHO. 

100      = CO level above which the Baltimore fire department requires the immediate evacuation of any building, although in most cases they usually order evacuations above 25ppm. 

100-300     CO range in which consumer CO alarms are required by EN50291 to sound but only if and when this level has been continuously sustained for 10 to 40 minutes.

150-400 +/-5 CO range in which consumer CO alarms are required by UL2034 to sound but only if and when this level has been continuously sustained for 10 to 50 minutes. 

200               = CO "ceiling" level above which US NIOSH recommends immediate evacuation of any workplace. 

300+ CO level above which consumer CO alarms are required by EN50291 standard to sound within 3 minutes. 

400 +/-10 CO level above which UL2034 require consumer CO alarms to sound but only if and when this level has been continuously sustained for 4 to 15 minutes.  There is no CO level at which UL2034 permits CO alarms to sound in less than 4 minutes.  

800 = CO level allowed from the "air-free" exhaust vents of gas ovens by a voluntary appliance industry standard of the American National Standards Institute (ANSI Z21) that has not been changed since it was adopted in 1925 when all ovens had flues.   Despite this standard, CO inspectors commonly find modern gas ovens that exhaust more than 800ppm, and even electric ovens may do so in "self-clean" mode when they stay on high for hours in order to burn away all food residues inside.  

1,200 = CO level US NIOSH deems “Immediately Dangerous to Life and Health” (IDLH).

3,000 = CO level repeatedly inhaled (and then held for 10 seconds before exhaling) in a lung function test called DLCO which measures the "diffusing capacity of the lung for carbon monoxide" (aka Transfer Factor).  The test uses an FDA-approved device for which no safety studies are available from any source, and for which no informed consent is required.     

10,000-40,000 = approximate range of CO in exhaust of gasoline engines without working catalytic converters or with cold catalytic converters, which take 30-60 seconds to heat up when engines are cold started, after which they reduce CO in exhaust to under 200ppm.

1,000,000 (pure CO) = CO level commonly used in 20th century (but now rarely) by veterinarians to euthanize animals in gas chambers.  CO exposure at this level kills any mammal in just a few breaths, without appreciably changing their COHb level. 

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