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Carbon Dioxide In Workplace Atmospheres

Related Information: Chemical Sampling - Carbon Dioxide
Method no.: ID-172
    
Matrix: Air
    
OSHA Permissible Exposure
Limits
    Final Rule Limits:
   
    Transitional Limit:


10,000 ppm Time Weighted Average (TWA)
30,000 ppm Short-Term Exposure Limit (STEL)
5,000 ppm TWA
   
Collection Procedure: Each sample is collected by drawing a known volume of air into a five-layer gas sampling bag.
   
Recommended Air Volume: 2 to 5 liters
   
Recommended Sampling
Rates
    TWA Determinations:
    STEL Determinations:


0.01 to 0.05 L/min (4 to 8 h sample)
0.3 L/min (15-min sample)
   
Analytical Procedure: A portion of the gas sample is introduced into a gas sampling loop, injected into a gas chromatograph, and analyzed using a thermal conductivity detector.
   
Detection Limits
    Qualitative:
    Quantitative:

200 ppm
500 ppm
   
Precision and Accuracy
    Validation Range:
    CVT:
    Bias:
    Overall Error:

2,000 to 10,000 ppm
0.026
-0.005
5.7%
   
Special Requirements: Samples should be analyzed within 2 weeks of collection.
   
Method Classification: Validated Method
   
Chemist: Rick Cee
   
Date (Date Revised): 1987 (June, 1990)
   

Commercial manufacturers and products mentioned in this method are for
descriptive use only and do not constitute endorsements by USDOL-OSHA.
Similar products from other sources can be substituted.
   
Branch of Inorganic Methods Development
OSHA Technical Center
Sandy, Utah


1. Introduction

This method describes the collection and analysis of carbon dioxide (CO2) in workplace atmospheres. Samples are collected in gas sampling bags and analyzed using a gas chromatograph (GC).
1.1. History

In the past, the OSHA Salt Lake Technical Center (OSHA-SLTC) method for analysis of CO2 consisted of a bicarbonate titration using phenolphthalein as the indicator (8.1). The past method suffered from a lack of specificity, possible contamination from ambient CO2, and a potentially unsafe collection method. Carbon dioxide was collected in impinger solutions containing sodium hydroxide.

The most recent OSHA method for measuring CO2 exposures in the workplace required the use of detector tubes (8.2). Because short-term detector tubes offer only spot checks of the environment, another method was needed to determine long-term CO2 concentrations. Other sources advocated the use of gas chromatography for CO2 analysis (8.3-8.5). This method is similar to the one proposed by NIOSH (8.3), with some modifications.

1.2. Principle

For time weighted average (TWA) or short-term exposure limit (STEL) determinations, a sampling pump is used to capture a known volume of air into a five-layer gas sampling bag. A GC fitted with a gas sampling loop and a thermal conductivity detector (TCD) is then used to assess sample concentrations of CO2.

1.3. Advantages and Disadvantages
1.3.1. The method is specific for CO2 in industrial environments. Response characteristics and the retention time for CO2 lead to positive identification. Mass spectrometry can be used if additional verification is necessary.

1.3.2. The method can be used for ventilation assessments in indoor air quality investigations. A CO2 level of 1,000 ppm is considered a determinant of ventilation system performance.

1.3.3. No impinger sampling is required; however gas sampling bags are used and may be somewhat inconvenient when handling and shipping.

1.3.4. Changes in humidity do not affect sample collection.

1.3.5. The bulk of the sample is not destroyed during analysis; other potentially toxic gases may also be analyzed from the same sample.

1.3.6. The gas bags are reusable.

1.3.7. The method requires the use of a GC with a gas sampling valve.

1.3.8. Analytical time required per sample is short; elution of CO2, using stated GC conditions, occurs within 5 min.

1.3.9. Gas bag samples are only stable for approximately 2 weeks. Samples should be analyzed as soon as possible.
1.4. Prevalence and Use

In 1979, CO2 was the 23rd largest volume chemical produced in the United States (8.6). Potential sources for CO2 emission and exposure are listed:

carbonated beverage manufacturing
carbonic acid manufacturing
fire extinguisher manufacturing
explosive manufacturing
municipal water treatment facilities
aerosol propellant manufacturing
breweries and fermentation plants
refrigeration units
cloud seeding
greenhouse air enrichment
lime kilns
by-product of ammonia production
product of combustion
anode baking products
fractioning and acidizing of oil wells
bakeries
grain elevators
silos
submarines
diving
mining
wells
shielded arc welding

1.5. Physical and Chemical Information (8.6, 8.7):

Molecular formula CO2
Molecular weight 44.01
CAS No. 124-38-9
Appearance colorless, odorless gas
Taste slightly acidic (from reaction with H2O to for carbonic acid)
Flammability non-flammable
Boiling gravity -78.5C (sublimes)
Specific gravity (air = 1) 1.5240 (0C)
Density 1.97 g/L (@ STP)
Synonymns carbonic anhydride
carbonic acid
gas dry ice

1.6. Toxicology


Information contained within this section is a synopsis of present knowledge of the physiological effects of CO2 and is not intended as a basis for OSHA policy.

Carbon dioxide is classified as an asphyxiant gas. In the atmosphere, CO2 normally exists at concentrations between 300 and 700 ppm. Larger gas-phase concentrations of CO2 may produce signs and symptoms of increased respiratory rate, lassitude, sleepiness, headache, convulsions, dyspnea, sweating, dizziness, or narcosis. Literature citations reveal a wide variation in physiological response to exposures at certain CO2 concentrations (8.6 - 8.10). Exposure to CO2 concentrations above 10% are generally agreed upon as posing an immediate physiologic threat (8.7 - 8.10).

Inhalation of CO2 can produce physiological effects on the central nervous, respiratory, and the cardiovascular systems. Central nervous system (CNS) effects vary with CO2 concentrations. Signs and symptoms of CNS involvement include lassitude, drowsiness, narcosis, and convulsions. At low levels, inhalation of CO2 may cause a mild depression of the CNS. At approximately 30% CO2 a paradoxical CNS stimulation leading to convulsions and coma is seen. Carbon dioxide concentrations above 50% induce an anesthetic effect (8.9).

Carbon dioxide is a potent stimulator of respiration.

Respiration depth and rate is mainly controlled through CO2 blood levels. Generalized signs of respiratory involvement are displayed by shortness of breath, dyspnea, respiratory acidosis, and a rapid increase in respiratory rate.

Cardiovascular effects of CO2 are demonstrated by generalized increases in blood pressure, vasodilation, heart rate, and cardiac output. Peripheral and cerebral vasodilation, as demonstrated by signs of sweating and headaches, are usually the first symptoms observed and are prevalent in low concentration exposures (8.7, 8.10).

The signs and symptoms of low level CO2 intoxication are sudden and reversible. Effects of intoxication are usually apparent within seconds of toxic exposure. After discontinuation of exposure, signs and symptoms usually dissipate within a few minutes.
2. Range, Detection Limit, and Sensitivity (8.11)
2.1. The analytical working range is linear from at least 200 to 30,000 ppm. The largest standard used during the study was 30,000 ppm; the response characteristics of the TCD indicate the upper linear limit may be much larger.

2.2. The qualitative detection limit was 200 ppm using a 1-mL sample loop. The quantitative detection-limit is 500 ppm. A lower detection limit for CO2 can be achieved using a larger gas-sampling loop; however, ambient CO2 levels are at least 300 ppm. Evaluation below 500 ppm would most likely be unnecessary for workplace atmosphere surveillance.

2.3. The sensitivity of the analytical method (using analytical conditions stated for a Hewlett-Packard 5730A Gas Chromatograph and 3385A Automation System) was taken from the slope of the linear working range curve (200-30,000 ppm range). The sensitivity is 1.771 area units per 1 ppm. (For the HP 3385A Automation System, 1 area unit = 3.2 microvolt-second.)
3. Method Performance (8.11)
3.1. The pooled coefficient of variation for the sampling and analytical method was 0.026. The variation was calculated from data within the range of 2,000 to 10,000 ppm.

3.2. The average recovery of generated samples taken in the 2,000 to 10,000 ppm range was 99.5%. The range of recoveries was from 93 to 104%.

3.3. Precision and accuracy data are derived from generated samples that were aged less than 2 days. The stability of CO2 in sampling bags is within precision and accuracy limits up to 14 days after sample collection.

3.4. Stability tests indicate a significant loss (>10%) of CO2 when samples are stored longer than 14 days. Samples should be analyzed as soon as feasible to minimize storage losses.
4. Interferences

The gas chromatographic determination of CO2 is relatively specific; however, any compound having a similar column retention time and response as CO2 is a potential interference.
4.1. Potential interferences may be minimized by altering operational conditions such as temperature and column packings or using gas chromatograph-mass spectrometry as a secondary source of confirmation.

4.2. Using the conditions stated within the method, other common gases and vapors do not present potential interferences. Nitrogen, hydrogen, oxygen, methane, and carbon monoxide retention times are significantly less than that of CO2. Chromatograms showing the elution of various common atmospheric gases and CO2 are shown in Figures 1 and 2.
5. Sampling
5.1. Equipment
5.1.1. A personal sampling pump capable of delivering a flow rate of approximately 0.01 to 0.05 L/min is necessary for TWA determinations. A larger flow rate pump can be used for STEL assessments. Either pump must have external inlet and outlet ports and hose barbs.

5.1.2. Five layer, 5-L aluminized gas sampling bags are used as the collection media (the bags can be obtained from OSHA-SLTC or Calibrated Instruments Inc., Ardsley, NY).

5.1.3. Various lengths of flexible tubing are used to make pump, sampling media, and breathing zone connections.
5.2. Sampling Procedure
5.2.1. Calibrate personal sampling pumps. Since the sampling bags have a total volume capacity of approximately 6 L, the following sampling scheme for TWA measurements is shown:

Flow rate (L/min)

Hours sampled

Total sample volume (L)


0.015 4 3.6
0.022 4 5.3
0.035 2.5 5.3
0.050 1.5 4.5


A large flow rate (0.040 to 0.050 L/min) will require placing new sampling bags into position throughout the day. For TWA determinations, a flow rate of approximately 0.020 to 0.025 L/min is sufficient for a 4 h sample. For STEL samples, calibrate the pump to a rate of approximately 0.3 L/min.

5.2.2. Evacuate and check gas sampling bags for leaks. The sampling bag can be evacuated and leak-tested by applying a vacuum to the bag. If a vacuum is applied to a leaky sampling bag, the bag will not fully collapse. If a vacuum pump is not available, gas sampling bags can be inflated, inspected for leaks, and then evacuated by hand rolling and flattening.

5.2.3. Label each sampling bag. Attach one end of a piece of flexible tubing to the inlet hose barb of the pump, and place the other end in the breathing zone of the worker. Use another piece of tubing to connect the metal valve sampling bib of the sampling bag to the outlet hose barb of the pump. A graphic representation of the pump set-up is shown:

For problems with accessibility in using figures please contact the SLTC at (801) 233-4900.
Text Version: The sampler is composed of an appropriate length of flexible plastic tubing attached to the inlet of a personal sampling pump. The other end of this flexible plastic tubing is placed in the breathing zone of the worker. Another appropriate length of flexible plastic tubing is used to connect the outlet of the sampling pump to the inlet of the sampling bag.


5.2.4. For personal sampling attach the gas sampling bag to any loose fitting clothing on the worker's back or side using tubing clamps.

5.2.5. When ready to sample, open the gas sampling bag valve by rotating the metal valve counter-clockwise until fully open. Attach the free end of the tubing connected to the bag to the outlet hose barb. Turn on the pump.

5.2.6. After sampling, rotate gas sampling bag valve clockwise until tight. Record the total air volume taken.

5.2.7. Do not prepare or submit blank samples. Request the laboratory analyze for carbon dioxide.

5.2.8. Wrap an OSHA 21 (or equivalent) seal across the gas sampling bag valve.

5.2.9. When submitting the sampling bags to the laboratory for analysis, pack loosely and apply generous padding to minimize potential damage during shipment. Submit samples as soon as possible after sampling.
6. Analysis
6.1. Precautions
6.1.1. When preparing CO2 standards, an Ascarite II filter is used to remove CO2 from the diluent air. Ascarite II contains sodium hydroxide and silica: Avoid contact with skin and mucous membranes. Wear gloves and safety glasses when handling Ascarite II.

6.1.2. The preparation of CO2 standards should be performed in a hood. Avoid breathing in any toxic concentrations of CO2.
6.2. Equipment
6.2.1. A gas chromatograph fitted with a 1-mL stainless steel gas sampling loop, sampling valves, and a TCD are used. Loops larger than 1-mL can also be used.

6.2.2. Standard preparation (8.11):

Due to stability considerations, use only five-layer aluminum gas sampling bags for standard generation. Gas sampling bags composed of Tedlar or saran can be used for standards provided they are prepared and analyzed within 24 h.

6.2.3. Columns:

Chromosorb 102, 6 ft 1/4 in. stainless steel, 80/100 mesh. Similar results were obtained using a 5 ft 1/4 in. stainless steel, 80/100 mesh, Porapak QS column.

6.2.4. Data reduction:

An electronic integrator is used to calculate peak areas.

6.2.5. Standard generation:

Certified CO2 standards can be used or standards can be prepared using any combination of: Calibrated gas-tight syringes or calibrated rotameters, mass flow controllers, or soap bubble flowmeters. A stopwatch is also necessary.

6.2.6. Gases:
  1. If certified standards are not available, undiluted, bottled CO2 (>99.8% purity) or pre-diluted CO2 can be used to generate gas standards. If pure liquid CO2 is used, a heating tape and variable transformer are necessary for regulator heating.

  2. Filtered, compressed, CO2-free air is used for dilutions. Ambient CO2 is removed from the compressed air using an Ascarite II/Drierite in-line trap. (See Precautions in Section 6.1 before handling Ascarite) Other methods, such as slaked lime traps, can also be used for removal of ambient CO2. A diagram of the Ascarite II trap and further information can be found in reference 8.11

  3. Helium (purified) is used as the carrier gas.
6.2.7. Additional accessories:

A personal sampling pump, with an inlet and outlet port and hose barbs, is used to load the gas sampling loop (loop loading can also be manually performed by squeezing the sampling bag).
6.3. Standard Preparation
6.3.1. Prepare standards by either calibrated syringe or metered injection of pure or diluted CO2 into the dilution medium. Please see precautions in Section 6.1.2. before preparing.

6.3.2. Completely evacuate and flush the gas bags used for standard preparation with CO2-free air (Note: The in-line trap with an Ascarite II/Drierite bed is used to remove any CO2). Use a soap bubble flowmeter to measure air flow rates immediately before and after diluent air addition.

A standard dilution scheme using pure CO2 is listed for 5-L gas bags:
ppm Standard

CO2 volume (mL)

Airvolume (mL)


Blank 0 4,000
1,248 5 4,000
2,494 10 4,000
4,975 20 4,000
9,901 20 2,000
19,608 40 2,000


Always prepare a blank standard from the compressed air to account for potential CO2 contamination. Other dilution schemes using smaller or larger gas bags and gas volumes can be used. Standards should be prepared in concentrations that bracket the sample concentrations.

6.3.3. For concentrations other than those listed above, use the following equation when using pure or pre-diluted CO2:

ppm CO2 = (mL CO2)(1 × 106)(A)
(mL diluent air + mL CO2)


where:
A = mole fraction or decimal per cent concentration of the pre-diluted mixture (i.e., for 1.93% CO2 stock standard, A = 0.0193. For pure CO2, A = 1).


6.3.4. If a metered generation of standards is desired, use a mass flow controller or calibrated rotameter to verify and control the CO2 delivery rate from a gas cylinder. Use a soap bubble flowmeter immediately before and after the standard generation to verify the CO2 flow rate. Meter a known amount of CO2 into a bag already containing a known volume of CO2-free air. Use a stopwatch to determine the volume of CO2 delivered over time.

6.3.5. If using calibrated syringe injection, fill a gas sampling bag with concentrated CO2 or use syringe extraction from an in-line gas cylinder septum. Most gas bags have injection ports or septa for gas syringe withdrawal or injection. Fill and flush a previously calibrated gas-tight syringe with pure CO2. Then withdraw and inject the required volume of CO2 into a gas bag already containing a measured amount of diluent air.
6.4. Analytical Procedure

6.4.1. Gas chromatograph conditions:

Helium carrier gas flow rate 15 to 25 mL/min
Reference gas flow rate 15 to 25 mL/min
Detector, manifold, and column temperature 20 to 25C
Retention time 2 to 6 min

6.4.2. Sample and standard introduction:
  1. Connect the outlet port of the personal sampling pump to the sampling loop via inert tubing.

  2. Adjust the pump to give a suitable flow rate for sample loading from the bag to the sampling loop.

  3. Connect a short piece of tubing from the inlet port of the pump to the sample bag. The bag valve is then turned counterclockwise to the open position and the pump is turned on.

  4. After the sample is loaded into the loop, turn the pump off to allow the loop sample to return to atmospheric pressure. Wait 1 to 2 min for pressure equalization and then open the gas sampling valve. Carrier gas flow is now directed through the sampling loop to the column and detector. (Note: Samples and standards can be introduced into the loop without a pump by simply squeezing a sufficient amount of sample from the bag into the loop. The sampling bag must be released for loop sample pressure normalization before opening the gas sampling valve.)

  5. Perform two determinations of each sample and standard.
6.4.3. If present in the sample, oxygen, nitrogen, carbon monoxide, and methane will elute before CO2. Examples of integrated chromatograms of CO2 and other common gases are shown in Figures 1 and 2.
7. Calculations
7.1. If blank correction is necessary for the standards, subtract blank peak area from standard area readings before constructing the concentration-response curve. No blank correction is necessary for the samples.

7.2. Calculate ppm CO2 concentrations from a linear least-square regression curve. Establish the regression curve using peak area (or heights) versus ppm. No calculations using air volumes are necessary since gas phase samples are compared directly to gas phase standards. Since the total capacity of the sampling bag is approximately 6-L, field air volumes can be used by the chemist to visually assess any leakage during shipment.

7.3. If necessary, the sample can be analyzed by gas chromatograph-mass spectroscopy to confirm the presence of CO2.

7.4. Report results to the industrial hygienist as ppm CO2.
8. References
8.1. Norton, J. F., ed.: Standard Methods for the Examination of Water and Sewage. 9th ed. New York, NY: America Public Health Association, 1946. pp. 33-40.

8.2. U.S. Dept. of Labor, Occupational Safety and Health Admin.: Chemical Information File. Online Database -- OSHA Information System. Washington, DC: Directorate of Technical Support, U.S. Dept. of Labor, OSHA, 1985.

8.3. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd ed., Vol. 3 (DHEW/NIOSH Pub. No. 77-157-C). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977. pp. S249-1-S249-6.

8.4. Katz, M., ed.: Methods of Air Sampling and Analysis. 2nd ed. Washington, D.C.: American Public Health Association, 1977. pp. 369-373.

8.5. Guiochon, G. and C. Pommier: Gas Chromatography in Inorganics and Organometallics. Ann Arbor, MI: Ann Arbor Science Pulishers Inc., 1973. pp. 80-115.

8.6. Hawley, C.G.: The Condensed Chemical Dictionary. 10th ed. New York, NY: Van Nostrand Reinhold Co., 1981.

8.7. National Institute for Occupational Safety and Health: Criteria for a Recommended Standard -- Occupational Exposure to Carbon Dioxide (DHEW/NIOSH Pub. No. 76-194). Cincinnati, OH: National Institute for Occupational Safety and Health, 1976. pp. 14-114.

8.8. Proctor, N.H. and J.P. Hughes: Chemical Hazards of the Workplace. Philadelphia, PA: J.B. Lippincott Co., 1978. pp. 147-148.

8.9. Goodman, L.S. and A. Gilman, ed.: The Pharmacological Basis of Therapeutics. 6th ed. New York, NY: Macmillan, 1980. pp. 331-334.

8.10. American Conference of Governmental Industrial Hygienists: Documentation of the Threshold Limit Values for Substances in Workroom Air. 3rd ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, 1976. pp. 296-298.

8.11. Occupational Safety and Health Administration Technical Center: Carbon Dioxide Backup Data Report) (ID-172). Salt Lake City, UT. Revised 1990.

Chromatograms of Nitrogen, Methane, and Carbon Monoxide Standards

For problems with accessibility in using figures please contact the SLTC at (801) 233-4900.
Figure 1


Chromatogram of a Mixture
 
Hydrogen 0.9972%
Oxygen 0.9974%
Nitrogen Balance
Carbon Monoxide 1.029%
Methane 0.9972%
Carbon Dioxide 0.9968%
RT AREA AREA %
0.21 1790 0.230
0.63 759200 97.688
1.18 7566 0.973
2.64 8768 1.128


For problems with accessibility in using figures please contact the SLTC at (801) 233-4900.
Figure 2