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For problems with accessibility in using figures and illustrations in this method, please contact the SLTC at (801) 233-4900.
These procedures were designed and tested for internal use by OSHA personnel. Mention of any company name or commercial product does not constitute endorsement by OSHA.
|OSHA Permissible Exposure
|1 ppm Time Weighted Average (TWA)*
3 ppm Short-Term Exposure Limit (STEL)*
||Passive badge monitor containing bisulfite-impregnated paper
|Recommended Sampling Time:
||8 h (4 to 16 h range)
|Average Sampling Rate:
||0.0614 ± 0.005 L/min (25 °C & 760 mmHg)
||Minimum 4.6 m/min (15 ft/min)
||A modified chromotropic acid procedure is used. Sample filters are desorbed using deionized water.
Solutions are acidified, and chromotropic acid is added. The color complex formed is analyzed using a UV spectrophotometer at 580 nm.
0.039 ppm (4-h sampling time)
0.11 ppm (4-h sampling time)
||0.8 to 72 ppm-h (as claimed by the manufacturer)
|Precision and Accuracy
| Validation Range:
||0.2 to 4.9 ppm
| Overall Error**:
|The 3M Model 3721 monitor is recommended for TWA determinations only. It is not recommended for STEL monitoring.
Any samples taken for STEL determinations should follow OSHA method No. 52.
|As compared to OSHA method no. ID-102.
James C. Ku
Commercial manufacturers and products mentioned in this method are for descriptive use only and
do not constitute endorsements by USDOL-OSHA. Although the following sampling procedure
uses a specific formaldehyde monitor, other passive monitors can be substituted provided they meet
Branch of Inorganic Methods Development
OSHA Technical Center
Salt Lake City, Utah
This method describes the passive monitor collection of airborne formaldehyde in the breathing zone of
workplace personnel and the subsequent analysis of those samples using a colorimetric technique.
Although this method specifically mentions the 3M Model 3721 monitor, other monitors can be used
provided performance requirements have been met. Some examples of validation procedures to
determine performance are given in references 5.1. and 5.2.
The simplicity and freedom of the 3M Model 3751 formaldehyde passive monitor showed
promise when first offered in 1981 as an industrial hygiene sampling alternative for
formaldehyde (5.3.); however, subsequent independent studies indicated analyte loss when
sampling at low humidities (5.4., 5.5.). Consequently, the Model 3751 monitor was removed
from the market by 3M in April, 1984. The Model 3721 3M monitor, capable of sample
humidification, was introduced in 1985 as a replacement. The changes instituted by 3M and
incorporated into the model 3721 are:
- A water-saturated pad in the bottom section of the monitor has been added for sample humidification.
- Each monitor is now packaged in a sealed metal container. Previously, the Model 3751 monitor was enclosed in a resealable plastic bag.
- The calculated sampling rate has been changed from 0.0659 to 0.0614 L/min.
Note: The sampling rate of 0.0614 L/min is in agreement with a previous OSHA Salt Lake City Analytical Laboratory (SLCAL) study (5.5.).
With the exception of the moisturizing pad, the appearance of the Model 3721 is physically
identical to the Model 3751 monitor. The Model 3751 monitor has been extensively evaluated
by independent laboratories (5.4.-5.6.). Results from these studies did not indicate serious
problems with desorption efficiency, face velocity, reverse diffusion, or post-collection sample
storage stability. The recent modifications instituted by 3M suggest sampling performance
would not be significantly affected in these areas. As long as the face velocity of the sampled
environment is above 4.6 m/min (15 ft/min), the sampling rate of the monitor does
not appear to be significantly altered (5.4.-5.7.). Sampling and analytical procedures are
identical for either model monitor; however, result calculations are different since slightly
different sampling rates are used.
The 3M formaldehyde monitor is a diffusion-type air monitoring assembly worn near the
breathing zone of personnel to evaluate potential exposure to formaldehyde (HCHO) vapors.
Formaldehyde vapor is adsorbed on bisulfite-impregnated paper located within the assembly.
The resulting adduct is desorbed with deionized water. An aliquot of the sample is reacted with
chromotropic acid in the presence of sulfuric acid to form a purple mono-cationic chromogen.
The absorbance of this colored solution is read in a spectrophotometer at 580 nm and is
compared to prepared standards. Although the chemistry of the color formation is not
well-established, the following reaction mechanism is proposed in acidic solution (5.8.):
1.3. Advantages and Disadvantages
1.3.1. This method has adequate sensitivity for measuring workplace atmosphere
concentrations of formaldehyde for TWA determinations.
1.3.2. The passive dosimeter used for collection of formaldehyde vapor is small, lightweight,
and requires no sampling pumps.
1.3.3. The collected formaldehyde sample is stable for at least 30 days.
1.3.4. One disadvantage of the method is that the analytical procedure may not be capable of
accurately determining STEL exposures at or below 3 ppm.
1.3.5. Another disadvantage with the dosimeter is sample rate dependence on face velocity.
The dosimeter should not be used in areas where the air velocity is less than 4.6 m/min
(15 ft/min). Most industrial work areas have air movement above 7.6 m/min (25
1.3.6. A disadvantage concerning the analytical procedure is the use of concentrated
H2SO4 during sample preparation.
Extreme care should be used when handling H2SO4.
1.4. Method Performance (5.5., 5.9.)
1.4.1. This method was validated over the range of 0.2 to 4.9 ppm.
1.4.2. The coefficient of variation (CVT) for the total analytical and sampling method
(50% RH) was 0.084. The overall error (as compared to the reference method OSHA
ID-102) was ±18.6%.
1.4.3. The qualitative detection limit of the analytical method is 0.7 µg of formaldehyde based
on a 3.0-mL sample volume. This is equivalent to 0.039 ppm for a 240-min sampling time.
1.4.4. The quantitative determination limit for the analytical method is 2 µg of formaldehyde
in a 3.0-mL sample volume. This is equivalent to 0.11 ppm for a 240-min sampling time.
1.4.5. Somewhat variable results were obtained when sampling for a short duration (STEL).
Therefore, the 3M Model 3721 monitor is recommended for 4 to 16-h sampling
measurements only, and is not recommended for STEL sampling.
1.4.6. The Model 3751 monitor was extensively evaluated in 1982 (5.5.) and included storage
stability, face velocity, sampling rate, and reverse diffusion experiments. Due to the
similarity of the 3751 and 3721 monitors, these experiments were not repeated for the
Model 3721. The 3751 experiments indicated (5.5.):
- The results of a storage stability test show that the mean recovery of samples stored after 30
days were within ±10% of the mean of monitors analyzed immediately after sampling.
- The results of a face velocity test indicate that the 3M Model 3751 monitor canaccurately measure
a known concentration as high as 10 ppm at face velocities as low as 15 ft/min.
- The results of a sampling rate validation test indicate that the average sampling rate was 0.0614 ± 0.005 L/min.
- The results of a reverse diffusion test indicate that reverse diffusion of collected formaldehyde from the monitor
back into the atmosphere should not be a significant factor when sampling over an 8-h sampling period.
1.5.1. When other substances are known or suspected to be present in the air, such
information, including their suspected identities, should be transmitted with the sample.
1.5.2. Any compound that has the potential of developing the same color as the
formaldehyde/chromotropic acid complex using the conditions described in this method
is an interference.
1.5.3. It has been reported by 3M that there is no interference from phenol (5.10.). The lack
of interference is mainly due to the monitor's inability to collect a significant amount of
1.6. Uses (5.11.)
1.6.1. Formaldehyde (CAS 50-00-0) is used mainly as a raw material for producing synthetic
resins. This accounts for over 50% of the total production of formaldehyde.
1.6.2. Potential occupational exposures to formaldehyde are listed:
Dress goods store personnel
Electrical insulation makers
Embalming fluid makers
Ethylene glycol makers
Formaldehyde resin makers
Furniture dippers and sprayers
Glue and adhesive makers
Grease-resistant textile finishers
Lacquerers and lacquer makers
Oil well workers
Photographic film makers
Textile mordanters and printers
1.7. Physical Properties (5.11.):
|Relative vapor density
||1.043 (air = 1)
||Soluble in water, alcohol, and ether
||Pungent and irritating
|Explosive limits (Gas)
||Gas 7.0-73% by volume in air
|Flashpoint (closed cup)
||50 °C (122 °F) of aqueous solution
|| Information listed within this section is a synopsis of current knowledge of the physiological effects of
formaldehyde (HCHO) and is not intended to be used as the basis for OSHA policy.
Formaldehyde is considered a strong irritant and potent sensitizer. Inhalation
of large amount of HCHO can cause severe irritation of the upper respiratory
tract and death. Data from human exposures indicate that exposure to large
concentrations of HCHO gas may lead to pulmonary edema. Even HCHO gas
present in the workroom at concentrations of 1 to 11 ppm can cause eye, nose,
and throat irritation (5.11.). Formaldehyde has the potential to cause cancer in
The following symptoms have been noted in some individuals (5.12.):
|0.5 to 2 ppm
||eyes, nose and throat irritation
|3 to 5 ppm
||tearing of the eyes
|10 to 20 ppm
||difficult breathing, nose and throat burning, cough, heavy tearing of the eyes
|25 to 30 ppm
||severe respiratory tract injury
||immediately dangerous to life and health (IDLH)
2.1.1. Avoid inhalation of or skin contact with formaldehyde.
2.1.2. If the possibility exists that the face velocity of an area being sampled is less
than 4.6 m/min (15 ft/min), an active sampling device (i.e. OSHA sampling
and analytical method No. 52) should be used instead of the passive monitor.
2.2. Equipment - Passive Monitors (If provided, also follow the 3M Formaldehyde Monitor Model
3721 - Instructions for Use.)
The 3M Model 3721 formaldehyde monitor (3M, St. Paul, MN) contains the following parts:
||Container consisting of two aluminum cans held together by a label. The two cans are labeled can A and can B.
||Can A contains:
Top Section (has a white film and plastic retaining ring),
Sealing Cup (has Date, Start Time, etc. written on it)
||Can B contains:
Bottom Section (has a metal clip attached),
Translucent Closure Cap
|| The original shipping container and aluminum cans can be reused for sample shipment to the lab.
2.2.1. Remove the plastic lid from can A. Open each can by grasping the ring tabs and
carefully pulling up. Remove the lids from both cans. Examine the contents to make
sure all parts are available.
2.2.2. Remove the Translucent Closure Cap from the Bottom Section. Save the Closure Cap.
2.2.3. Pressing firmly, snap together the Top and Bottom Sections. Make sure the white film
and plastic ring are NOT removed from the Top Section. The monitor is now ready
2.3. Sampling Procedure
2.3.1. Immediately begin sampling by attaching the monitor to the employee or by placing it
in the sampling area. The white film (Top Section) should face away from the
2.3.2. Record the following information:
1) Beginning sampling time
2) Sampling date
3) Monitor serial number
4) Employee or area identification
5) Temperature, pressure, and relative humidity at the sampling site
2.3.3. If possible, sample for 8 h. The minimum sampling time recommended is 1 to 2 h.
For indoor air quality investigations, sample up to 16 h.
2.3.4. Immediately after sampling, remove and discard the white plastic film and purple
retaining ring from the monitor. In place of the film/ring, snap on the Translucent
Closure Cap by applying some pressure. A "clicking" sound should be heard when the
cap is securely fastened.
2.3.5. Be sure both plugs on the Translucent Closure Cap are firmly seated. This will insure a
2.3.6. Snap the Sealing Cup into place on the Bottom Section of the monitor. Be sure the cup
is snapped securely.
2.3.7. Record the end sampling time and any drastic change (>10%) in temperature,
pressure, or relative humidity that may have occurred during sampling.
2.3.8. Assemble a blank sample in the same fashion as mentioned in Sections
2.2.1.-2.2.3. and 2.3.4.-2.3.6. Do not expose the blank.
2.4. Sample Shipment
2.4.1. Place each monitor back into the aluminum container, cover with the plastic cap, and
securely wrap each can with an OSHA Form 21 sample seal.
2.4.2. Submit at least one blank sample with each set of samples. The blank sample should
have been handled in the same manner as the other samples except that it was not
exposed. If possible, also submit a "lot blank". This is an unused monitor inside an
unopened aluminum container.
2.4.3. When other substances are known or suspected to be present in the air, such
information should be transmitted with the sample.
2.4.4. Send the monitors directly to the laboratory and request formaldehyde analysis. The
original shipping carton can be used for shipment.
3.1.1. Refer to instrument manuals for proper operation.
3.1.2. Observe laboratory safety regulations and practices.
3.1.3. CAUTION: Sulfuric acid can cause severe burns. Wear protective gloves, labcoat,
and eyewear when handling concentrated sulfuric acid and the formaldehyde stock
CAUTION: Formaldehyde has the potential to cause cancer in humans (5.12.).
Extreme care must be observed when handling.
3.1.4. Do not store formaldehyde standards or samples in a refrigerator since polymerization
will occur. Polymer precipitation may be observed by the appearance of a white milky
substance in the formaldehyde solution.
3.1.5. Sodium sulfite solutions used for formaldehyde standardization gradually absorb carbon
dioxide on exposure to air. Solutions which have stood for more than a week should be
3.1.6. Do not use reagent bottles having caps which contain phenolic resins. Formaldehyde
contamination could occur.
3.2.1. Spectrophotometer: double beam, 1-cm cell.
3.2.2. Meter, pH.
3.2.3. Miscellaneous volumetric glassware or plasticware: Volumetric burets, graduated
cylinders, pipettes, volumetric and Erlenmeyer flasks, other laboratory glassware,
syringes. (Note: All glassware or plasticware should be washed and rinsed thoroughly
with deionized water and then air dried prior to use.)
3.2.4. Analytical balance (0.01 mg).
3.3. Reagents (All chemicals should be reagent grade or better.)
3.3.1. Deionized water (DI H2O).
3.3.2. Chromotropic acid sodium salt
solution (1%): Dissolve 1 g of chromotropic acid sodium salt (1,8-dihydroxy-3,6-naphthalenedisulfonic
acid sodium salt) in 100 mL of DI H2O. Prepare this solution daily.
(Note: This reagent is also commonly referred to as 4,5-dihydroxy-2,7-naphthalenedisulfonic acid sodium salt)
3.3.3. Sulfuric acid (H2SO4), concentrated.
3.3.4. Sodium bisulfite (NaHSO3), 1%: Dissolve 10 g of NaHSO3 in
1 L of DI H2O.
3.3.5. Formaldehyde (HCHO) solution, 37%.
3.3.6. Formaldehyde stock solution, ~1,000 g/mL: Dissolve 2.7 g (about 3 mL) of 37%
HCHO solution in 1 L of DI H2O. Standardize this solution as
described in Section 3.4. The solution is stable for at least 6 months.
(Note: After 6 months, the standardization should be repeated).
3.3.7. Reagents for standardization of HCHO stock solution:
- Sodium carbonate (Na2CO3),
certified, 99.9% minimum purity: Dry the Na2CO3
powder at 120 °C for 2 h, then transfer to a desiccator and cool to a constant weight.
Use as a primary standard.
- Sulfuric acid, 0.1 N: Dilute 3 mL of concentrated H2SO4
slowly to 1 L with DI H2O.
- Sodium sulfite (Na2SO3), 12.5% (W/V): Dissolve 140 g
of anhydrous Na2SO3 in 980 mL
DI H2O. Store in a refrigerator (approximately 4 °C).
3.4. Standard Preparation
3.4.1. Standardization of the HCHO ~1,000 µg/mL stock solution (5.13., 5.14.):
- Standardize the 0.1 N H2SO4 solution using the certified Na
2CO3 as a primary standard: Weigh 1.00 to 1.20 g of dried Na2
CO3 into a 250-mL beaker containing 50 mL of DI H2O, add 3 drops of methyl
red/bromocresol green indicator and titrate with the H2SO4 to a faint pink color.
Heat the titrated solution to a gentle boil for 2 min to expel any dissolved CO2, then cool the flask contents
to room temperature. If the end point has not been overrun, the indicator will reassume its characteristic green color. Complete the
titration with H2SO4 to a sharp color change. Calculate the normality of the H
2SO4 solution (N2) based on the following equation:
N2 = meq of Na2CO3/V2
V2 = mL of H2SO4 solution required to titrate the
- Use a pH meter and adjust the pH of 25.0 mL of the 12.5% Na2SO3 solution to 9.6 with
the standardized 0.1 N H2SO4.
- Place 50.0 mL of the HCHO ~1,000 µg/mL stock solution into a 250-mL beaker.
- Add the previously adjusted Na2SO3 solution to the 250-mL beaker and titrate to a
pH of 9.6 with the standardized 0.1 N H2SO4. Calculate the concentration of HCHO as
|HCHO, µg/mL =
A = mL of H2SO4 solution required to titrate the sample
B = mL of H2SO4 solution required to titrate the blank
C = normality of the H2SO4 solution (meq/mL)
D = (30 mg/meq of HCHO)(1,000 µg/mg)
= 30 × 103 µg/meq of HCHO
E = mL of formaldehyde used
3.4.2. Preparation of standards
To a series of 25-mL Erlenmeyer flasks already containing 2 mL of 1% NaHSO3,
carefully add 1.0, 3.0, 5.0, 10.0, 15.0, and 20.0 µL of the ~1,000 µg/mL HCHO stock
solution. If the stock solution is prepared as exactly 1,000 µg/mL HCHO after
standardization, these aliquots are equivalent to 1.0, 3.0, 5.0, 10.0, 15.0, and 20.0 µg
of HCHO. As an alternative, standards can be prepared in 1% NaHSO3 using serial
dilution of the ~1,000 µg/mL stock solution.
3.5. Sample Preparation
3.5.1. Assemble and prepare a "lot blank" for analysis, if available (also see Section 2.4.2.).
3.5.2. Open both ports of the Translucent Closure Cap of each monitor.
3.5.3. Using the center port of the Translucent Closure Cap and a small pipette or syringe,
add 3 mL of DI H2O to each monitor. Reseal the ports.
3.5.4. After 30 min, with occasional gentle agitation, transfer a 2-mL aliquot of the solution
into a 20-mL screw-cap glass vial and reserve for color development.
3.6.1. Develop the color of samples, standards, and blank solutions by adding 1 mL of 1%
chromotropic acid solution, and after thorough mixing, 5 mL of concentrated H2SO4.
(Note: Add the sulfuric acid slowly and carefully. Add H2SO4 to
the samples and standards in the same fashion since heat catalyzes the color formation.)
3.6.2. Allow the solutions to cool to room temperature, then measure the absorbance of each
solution at 580 nm using a 1-cm cell.
3.6.3. If the sample absorbance is larger than the absorbance of the highest standard, take a
smaller aliquot from the monitor, dilute to 2 mL, and repeat Sections 3.6.1.-3.6.2. Use
the appropriate dilution factor in calculations if an aliquot other than 2 mL is taken.
3.7.1. Use a least squares regression program to plot a concentration-response
curve of peak absorbance versus the amount (µg) of formaldehyde in each standard.
3.7.2. Determine the amount (µg) of formaldehyde, A, corresponding to the absorbance in
each analyzed sample aliquot from this curve.
3.7.3. Calculate the total amount (µg) of formaldehyde, W, in each sample:
||(A)(sample vol, mL)(DF)
DF = Dilution Factor (if none, DF = 1)
3.7.4. Blank correct each sample and calculate the concentration of formaldehyde in each
|ppm formaldehyde =
||(W - Wb) × MV
MW × (AV)
AV = ST × 0.0614 × (T1 / T2)1.5 × (P2 / P1)
Wb = Total µg of formaldehyde in the blank sample
MV = Molar volume at 25 °C and 760 mmHg (24.45 L/mole)
MW = Molecular weight of formaldehyde (30 g/mole)
ST = Sampling time (min)
0.0614 = Sampling rate (L/min) at 25 °C and 760 mmHg
T1 = Sampling site temperature (K)
T2 = 298 K
P1 = Sampling site pressure (mmHg)
P2 = 760 mmHg
3.8. Reporting Results
Report results to the industrial hygienist as ppm formaldehyde.
4. Backup Report
See Reference 5.9. for complete information.
5.1. Cassinelli, M.E., R.D. Hull, J.V. Crable, and A.W. Teass: Protocol for the Evaluation of
Passive Monitors. In Diffusion Sampling, An Alternative Approach to Workplace Monitoring,
edited by A. Berlin, R.H. Brown, and K.J. Saunders. London: Royal Society of Chemistry,
1987. pp. 190-202.
5.2. Occupational Safety and Health Administration Analytical Laboratory): "Precision and
Accuracy Data Protocol for Laboratory Validations" or "An Outline for the Evaluation of
Organic Sampling and Analytical Methods". In The OSHA Laboratory Methods Manual.
Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub No. ISBN:
5.3. Rodriguez, S.T., P.B. Olson, and V.R. Lund: "Colorimetric Analysis of Formaldehyde
Collected on a Diffusional Monitor." Paper presented at Amer. Ind. Hyg. Assoc. Conference,
Portland, OR, May 1981.
5.4. Kennedy, E.R. and R.D. Hull: Evaluation of the Du Pont Pro-Tek Formaldehyde Badge and
the 3M Formaldehyde Monitor. Amer. Ind. Hyg. Assoc. J. 47:94-105 (1986).
5.5. Occupational Safety and Health Administration - Salt Lake City Analytical Laboratory
(OSHA-SLCAL): Evaluation of 3M Formaldehyde Monitors (Model 3751) by J.C. Ku
(USDOL/OSHA-SLCAL Product Evaluation no. ID-139). Salt Lake City, UT. 1982
5.6. National Council of the Paper Industry for Air and Stream Improvement Inc. (NCASI): A
Laboratory Evaluation on the Performance of Passive Diffusion Badge Monitors and Detector
Tubes for Determination of Formaldehyde. (Technical Bulletin No. 451). NY: NCASI, 1985.
5.7. Occupational Health and Safety Products Division/3M: 3M Brand Formaldehyde Monitor
#3750/3751. St. Paul, MN: 3M Company, Internal document - No publication date given.
5.8. Feigl, Fritz: Spot Tests in Organic Analysis. 7th Ed. NY: American Elsevier Publishing Co.,
5.9. Occupational Safety and Health Administration Technical Center: Evaluation of 3M
Formaldehyde Monitors (Model 3721) by J.C. Ku and E.F. Zimowski (USDOL/OSHA-SLTC
Product Evaluation No. 10). Salt Lake City, UT. 1989.
5.10. 3M Company: Research Report for 3M Formaldehyde Monitor, St. Paul, MN: 3M Company,
Internal document - No publication data given.
5.11. National Institute for Occupational Safety and Health: Criteria for a Recommended Standard
- Occupational Exposure to Formaldehyde. (DHEW/NIOSH Pub. No. 77-126). Washington,
D.C., U.S. Dept. of Health, Education and Welfare, 1976.
5.12. "Formaldehyde" Code of Federal Regulations 29CFR 1910.1048. 1989. pp 315-351.
5.13. Blaedel, W.J. and V.W. Meloche: Elementary Quantitative Analysis. New York, NY.:
Harper & Row, 1963. pp. 366
5.14. Burlington Industries: Standard Test Method for the Determination of Latent Formaldehyde,
Burlington Industries Chemical Division, Internal document - No publication data given.