Carbon Monoxide -


CO EXPOSURES AND SCALE OF EFFECTS FROM ZERO TO ONE MILLION PARTS PER MILLION (ppm)
(Modified from Donnay, 2001)


<<<1 ppm (20-30 nanomoles/L tissue water; 0.56 ul/L) - Naturally/normally found in human tissues; a concentration 3-4 orders of magnitude smaller than would be possible based on its solubility in water (at body temp. & 760 mmHg max. solubility = 18.3 ml/L.). CO appears to play a physiological role in vasomotion, etc.

<1 ppm - Amount of CO in clean air.

0.4 - 1.6 ppm - Range of average ambient outdoor CO exposure over which asthma prevalence among middle schoolers was linearly correlated, and more signifantly than with any other pollutant studied (NOx, SOx, Ozone, or particulates of any size) (Hajat et al., Thorax, 1999; Norris et al., Environ. Health Perspectives, 1999).

1 ppm = 0.0001% - Increase in average ambient outdoor CO was associated with a 30% increase in the odds of unmedicated asthmatic children reporting symptoms the next day - stronger association than with any other pollutant studied: (Environ. Health Perspectives, Dec., 2000).

See the page on relationship between ppm and % of CO

0 - 2 ppm - Range of CO found in end-tidal breath (ETCOb) of healthy non-smokers due to systemic but variable endogenous CO production.

2 ppm (+/-2.5) - Level of CO found in U.S. homes.

3 - 4 ppm - Borderline range for ETCOb in non-smokers.

5 ppm - Lowest level of CO displayed by first-AIM low-level CO monitors (not approved by UL or CPSC).

3 - 7 ppm - A 6% increase in the rate of admission of non-elderly for asthma was associated with a change in CO in Seattle, Washington (Sheppard et al., Epidemiology, 1999).

>5.5 ppm (3-month average) - CO level above this value during the last trimester of pregnancy was associated with a significantly increased risk for low birth weight in Los Angeles study of 125,573 pregnancies (Ritz & Yu, Environ. Health Perspectives, 1999).

5 - 10 ppm - Range of increase in average outdoor CO found associated with a significant increase in heart disease deaths and hospital admissions for congestive heart failure.

5 - 25 ppm - Normal range of ETCOb in smokers who have not recently smoked.

9 ppm - US EPA's national ambient air quality standard 8-hour average (TWA) limit for CO exposure outdoors (rarely ever exceeded now in U.S. cities).

  • WHO 8-hour average (TWA) limit for CO exposure outdoors.
  • ASHRAE recommended indoor CO limit.
  • "Action" level with regard to CO for many public safety organizations.
  • Level above which many public safety organizations (fire departments, etc.) red-tag and prevent further use of combustion devices.
  • Lowest CO level producing significant effects on cardiac function (ST-segment changes, angina) during exercise in subjects with coronary artery disease. (Allred et al., Environ. Health Persp., 1991, and others).

    10 ppm = 0.001%

    11 ppm - Health Canada's 8-hour average legal limit for any CO exposure.

    15 - 35 ppm - Range of ETCOb found in non-smokers recently and chronically exposed to high levels of CO inddors from inadequately vented combustion appliances in their homes.

    20 ppm - US OSHA heat exposure standard: maximum allowable ETCOb post shift in non-smokers.

    25 ppm - WHO 8-hour average limit for any CO exposure.

    25 ppm - 150 ppm - ETCOb of smokers immediately after smoking.

    30 ppm - Lowest CO level that US CPSC and UL/CSA allow home CO alarms to display (based on UL standard #2034, 3rd revision, OCT., 1998). The health reasons for this are obscure.

    35 ppm - US EPA's national ambient air quality standard 1-hour average limit for CO exposure.

  • US NIOSH recommended 8-hour average limit for occupational CO exposure
  • Level above which Baltimore City fire fighters use self-contained breathing apparatus.

    50 ppm = 0.005% CO - US OSHA 8-hour average legal limit for occupational CO exposure (the highest occupational CO limit in the world)

    70 ppm - Lowest CO level at which US CPSC and UL/CSA allow home CO alarms to alarm, but only after 1 - 4 hours of exposure. The health reasons for this are also obscure.

    100 ppm = 0.01% CO - Level at which Baltimore City F.D. orders evacuation of any building (12-14% COHb at equilibration). many safety agencies will often do so at far lower CO concentrations.

    100 - 1,000 ppm = 0.01%-0.1% CO - Range of CO found in exhaust of gasoline-powered motor vehicles with hot (working) catalytic converters.

    200 ppm - Level at which US NIOSH recommends immediate evacuations of any building - level above which US CPSC and UL approved home CO alarms must sound after 30-60 min. - Level of CO (air-free) allowed inside water heater flues by an American gas association (now ANSI) standard.

    200 - 300 ppm - Range of CO in exhaled (diluted) cigarette smoke (exceeds water heater limit!).

    400 ppm - Level above which CPSC and UL approved home CO alarms must sound after 5-15 min.

  • Level of CO (air-free) allowed inside furnace flues by an American gas association (now ANSI) standard.

    500 ppm = 0.05% CO - Roughly the level of CO thought to be lethal in humans exposed over a period of many hours (42-45% COHb at equilibration).

    800 ppm - (air-free) Level of CO allowed inside oven flues by an American gas association (now ANSI) standard, # Z21.1. [this level is unchanged since 1921 when oven flues were still vented outdoors like woodstoves, instead of directly into the kitchen as they have since 1950s!

    1,000 ppm = 0.1% CO - Level of CO commonly considered rapidly lethal in humans (approx. 62% COHb at equilibration).

    1,000 - 5,000 ppm - Level of CO in exhaust gases from "well-tuned" diesel engines.

    1,200 ppm = 0.12% CO - Level of CO declared by US NIOSH to be immediately dangerous to life and health (potentially lethal within minutes, certainly within a few hours).

    3,000 - 10,000 ppm - Level of CO in exhaust gases from "well-tuned" propane-powered forklift trucks (ie. hilos) without functioning catalytic coverters. Dangerous!

    5,000 ppm = 0.5% CO - CO level in coal gas used almost exclusively for gas lighting in the USA from 1819-1865 (thereafter until widespread introduction of natural gas in the 1950's, coal gas was usually mixed with other gases containing even higher concentrations of CO).

    10,000 - 100,000 ppm = 1% - 10% CO - Range of CO found in exhaust of gasoline-powered vehicles without catalytic converters or with converters that are cold or otherwise not working. Dangerous!

    220,000 - 300,000 ppm - CO concentration undiluted blast furnace (steel-making) gases.

    250,000 - 350,000 ppm = 25% - 35% CO - Range of CO in mixed or utility gas made from 1870s until 1940s by mixing coal gas (5% CO) and water gas (50% CO).

    500,000 ppm = 50% CO - Water gas (made by spraying water onto hot coals, C: H2O + C = H2 + CO). Invented in Paris, but banned there in 1854 as too deadly. Introduced in the USA in 1858 and widely used in mixtures with coal and/or other gases until 1940s for lighting, heating, cooking and in industry. Banned only in Massachusetts from 1880 - 1890.

    1,000,000 ppm = 100% CO - pure CO: one breath is instantly lethal; at this level, CO kills neurologically, not via hypoxia, and COHb remains normal.


    ...... last changed 07/12/02



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