Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at

April 8, 1991



                   OFFICE OF FIELD PROGRAMS

SUBJECT:            Use of Blood Carboxyhemoglobin to Document Employee
                   Exposure to Carbon Monoxide

In response to your request concerning the correlation between carboxyhemoglobin (COHb) levels and occupational exposures to carbon monoxide (CO), the Directorate of Technical Support has provided us with the attached information.

Based on their literature review and expert consultation, they concluded that it is not presently feasible to precisely quantify workplace CO exposure levels based on blood COHb levels. However, conservative estimates may be made if certain criteria apply:

1. The employee is a non-smoker.

2. The work environment is methylene-chloride free.

3. Duration of the employee's exposure is sufficient to reach the steady state (horizontal) portions of Figure 2 ("Absorption of Carbon Monoxide") in the NIOSH Criteria Document on Carbon Monoxide.

In work situations which deviate from the above criteria, citations will be handled on a case-by-case basis, in consultation with the national office and expert opinions.


March 25, 1991

                   Directorate of Compliance Programs

THRU:               THOMAS J. SHEPICH 
                   Directorate of Technical Support

                   THOMAS WILCOX, M.D. Acting Director 
                   Office of Occupational Medicine

FROM:               CHARLES ALEXANDER, M.D., M.P.H. 
                   Medical Officer 
                   Office of Occupational Medicine

SUBJECT:            Use of Blood Carboxyhemoglobin Levels to Document
                   Employee Exposure to Carbon Monoxide

In response to your request concerning the correlation of carboxyhemoglobin (COHb) levels and workplace carbon monoxide (CO), our staff has reviewed the literature on the subject and consulted a number of experts in the field.

Peterson and Stewart published data in 1972 from which NIOSH extracted the figure to which you referred (Enclosure 1). This figure is attributed incorrectly to a preceding article by Stewart, et al. A copy of this paper is enclosed (Enclosure 2). To derive the curves they utilized a CO absorption and excretion formula devised in 1965 by Coburn, et al. Their validation of these curves was based upon COHb levels obtained in a controlled environment from non-smoking sedentary males between the ages of 18 and 42. Since the time required to reach steady state equilibration lessens with increased respiratory rates, it is likely that people actually working (with increased respiratory rates) will reach steady state COHb levels more quickly. Thus, the only portions of the curves that can be relied upon to extrapolate environmental CO levels are the steady state (horizontal) regions which require 6 to over 8 hours of CO exposure in sedentary males.

As you know, there are many variables which affect COHb levels, several of which you asked about. Unfortunately, there is no formula which takes all these critical variables into account.

Smokers have "baseline" COHb levels that range from 4% to reportedly as high as 20%, with the average probably in the 4-6% range. Non-smokers have much lower values, generally in the 0.2 to 2% range. Using Peterson and Stewart's figure, a smoker's COHb of 5% corresponds to an 8 hour exposure to 35 PPM, the PEL. It would be an error to ascribe this COHb blood level to occupational CO exposure in a smoker.

Human metabolism of methylene chloride causes formation of COHb. Thus, worker exposure to it will result in an additional increase in the measured blood COHb that would not be attributable to workplace CO exposure. Again, there is no formula available to accurately account for this.

Oxygen administration shortens the half-life of COHb. One hundred percent 02 at 1 atmosphere reduces the half-life from about 320 minutes to about 80. One hundred percent oxygen under 3 atmosphere's pressure administered in a hyperbaric chamber, will reduce the COHb half-life to about 25 minutes. The length of 02 administration, the patient's ventilatory parameters (e.g. volume inspired/minute) and the percent 02 actually received by the patient (as opposed to delivered to the patient) will affect blood COHb concentration.

Short of precisely knowing the worker's baseline COHb (prior to exposure), length of exposure to CO, ventilatory parameters, time between end of CO exposure and blood drawing, and length of administration time and percent 02 actually received by the patient, a quantifiable airborne CO workplace concentration cannot be determined. In addition, methylene chloride exposure is a separate variable for which no workable computation is available.

It is our opinion that any attempt to precisely quantify a workplace airborne CO exposure level based on a blood COHb level is not presently feasible. In a non-smoking employee, however, it would be possible to make a conservative estimate of the minimum workplace airborne CO concentration if the blood COHb level and the length of time from the end of exposure to the drawing of blood were known. This estimate would assume that no 02 was administered and that the employee had been exposed for a time sufficient to reach steady state COHb equilibrium. If the individual had been exposed for less than this period, the CO exposure would be higher than that estimated, but how much higher could not be precisely determined.

For example, if a non-smoking worker had a blood COHb of 17% drawn one hour after removal from a methylene chloride-free work environment, regardless if 02 were administered, a reasonable assumption could be made that the workplace CO level was at or above 100 ppm. This level is well in excess of the PEL. Exactly how much above could not be calculated. If the same non-smoking worker had a blood COHb level of 8% two hours after exposure, again, it would be reasonable to assume that the workplace CO level was above 50 ppm which exceeds the PEL. Even ignoring normal metabolism which slowly lowers the blood COHb level, either of these blood COHb values plotted on the steady state portion of the graph indicate CO airborne exposure levels sufficiently above the 8 hour TWA PEL that the likelihood of a workplace airborne CO level in compliance would be extremely low. If the worker is a smoker, the graph cannot be used to make reasonable assumptions regarding workplace CO levels since the baseline COHb would be uncertain.

Thus, figure 2 from the NIOSH criteria document can still be utilized in many cases to provide an approximate indicator of the magnitude of the problem, but it should not be depended upon to produce a precise extrapolated CO level. Environmental sampling remains the gold standard.

The Office of Occupational Medicine will continue to monitor the literature regarding this problem and will advise the regions should a workable computation method be devised.

January 30, 1991

TO:        Patricia Clark, Director 
          Directorate of Compliance Programs

THRU:      Leo, Carey, Director 
          Office of Field Programs

FROM:      Michael G. Connors 
          Regional Administrator

SUBJ:      Use of Blood Carboxyhemoglobin Levels to Document Employee
          Exposure to Carbon Monoxide

Our Region frequently investigates apparent employee exposures to carbon monoxide and routinely obtains blood carboxyhemoglobin (COHb) levels during the course of these inspections. In most inspections accurate air levels of carbon monoxide cannot be obtained because the inspectors arrive on site after the fact. Please provide guidelines on the use of COHb levels to support citations for overexposures to carbon monoxide. Generally our compliance staff uses Figure 2 in the NIOSH Criteria document on Carbon Monoxide, 73-11000, to calculate airborne exposures. It would be particularly helpful to our compliance efforts if you could clarify the following issues.

- What is the most accurate method of calculating air concentrations of CO from COHb levels?

- What modifications to calculations should be made for COHb levels taken under these conditions:

- for employees who smoke?
- for employees exposed to methylene chloride?
- for COHb levels taken more than 15 minutes after exposure?
- for employees administered oxygen before the blood sample was drawn?

If you need further information or clarification on any of these issues please contact Leslie Ptak of my staff at FTS 353-2220. Your assistance in this matter is greatly appreciated.

Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.