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Isocyanate Exposure, Reaction and Protection

Quick Tips #233
Introduction

Isocyanates are widely used and found in many industries and occupations, including painting, construction, shipbuilding, upholstery manufacturing and firefighting. Isocyanates have been used in the United States since the 1950s, and are produced by reacting a primary aliphatic or aromatic amine dissolved in a solvent such as xylene or monochlorobenzene with phosgene dissolved in the same solution. They contain two OASH-NCO cyanato groups attached to an organic radical, and react exothermically with compounds containing active hydrogen atoms to form a polymeric mass (polyurethane). This polyurethane is then used in the production of rigid or flexible foams, surface coatings, paints, electrical wire insulation, adhesives, rubbers and fibers.

The most common forms of isocyanates are toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) and Hexamethylene Diisocyanate (HDI).
TDI is a liquid at room temperature, and can cause asthma-like conditions when inhaled as an aerosol (such as spray paint). TDI is popular for producing many paints and coatings, along with flexible foam, which is used in making cushions for automobiles, furniture and mattresses.

MDI is used in the manufacturing of rigid foams, and must be heated before causing asthma-like conditions when inhaled as an aerosol. This makes MDI somewhat less hazardous than TDI, so it has been replacing TDI in certain applications. MDI is commonly used in the production of adhesives, automobile bumpers, shoe soles, coated fabrics and spandex fibers. It can also be found in paints.
HDI is mainly used to make polyurethane foams and coatings it is also used as a hardener in in automobile and airplane paint. Exposure can cause an allergic asthma-like response with coughing, wheezing and shortness of breath.

Some less common forms of isocyanates include:

  • napthylene diisocyanate (NDI)
  • polymethylene bisphenylisocyanate (PAPI)
Isocyanate Exposure Levels

The OSHA permissible-exposure limit (PEL) for TDI and MDI is 0.02 ppm of air as a ceiling limit. The ceiling is the highest concentration to which an employee can be exposed. The American Conference of Governmental Industrial Hygienists (ACGIH) recognizes 0.005 ppm as its threshold-limit value (TLV) as an eight-hour time-weighted average and 0.02 ppm as a short-term exposure limit (STEL) for TDI, MDI and HDI.

Air Monitoring for Isocyanate

OSHA test method 42 (for TDI and HDI) and method 47 (for MDI) spell out personal-monitoring procedures for isocyanates. Samples are to be collected by drawing a known volume of air through glass fiber filters with a recommended air volume and sampling rate of 15L at 1L to 2L per minute.

You can also conduct continuous isocyanates monitoring. Many companies offer single-point monitors that can continuously monitor isocyanates for up to one month. They operate by an electro-optical sensing system, which uses a cassette-like tape. A stain occurs on the tape, and is then read in proportion to the concentration of the isocyanate.

Different cassette tapes are available. Standard-play tapes are replaced every two weeks. Extended play tapes last for a month. Datalogging monitors with alarms are also available. These types of monitors are ideal in spray-booth operations.

Effects of Isocyanate Overexposure

Exposure to isocyanates can lead to chemical bronchitis and pneumonitis. An isocyanate reaction often includes coughing, tightness of the chest, shortness of breath, nausea, vomiting, eye and skin irritations, gastric pain and loss of consciousness.

Continuous overexposure to isocyanates can lead to pulmonary sensitization or "isocyanate asthma." When this occurs, symptoms improve when the irritant is removed. However, acute asthma attacks occur on renewed exposure, even when the encounter is very brief or at low levels of isocyanates, and can cause death.

Skin contact can cause inflammation and necrosis, which might lead to dermatitis. Wash hands with soap and water immediately upon contact.

Personal Protective Equipment for Handling Isocyanates

Prior to OSHA’s revision to the respiratory protection standard (April 8, 1998) supplied air respirators were required to help reduce exposures to isocyanates, this was appropriate due to the poor warning properties of isocyanates.  Now air purifying respirators may be used for those compounds that have poor warning properties if the cartridge change schedule is set up.  This is because cartridge change schedules are required instead of workers relying on warning properties of compounds for cartridge change out.  Properly selected and used air-purifying respirators can be used to safely and effectively to reduce exposures to common diisocyanates. Appropriate cartridge change schedules should be developed to ensure cartridges are changed before breakthrough occurs. OSHA allows employers to choose air-purifying respirators for diisocyanates if they are appropriate for their workplace.  A complete respiratory protection program per 29 CFR 1910.134 is necessary to ensure that respirators are selected properly and provide appropriate protection.


Isocyanates are also a hazard to the skin, hand protection such as Butyl rubber gloves or SilverShield®/4H gloves can adequately protect hands from isocyanates.  Chemical protective clothing that is rated for use to protect against isocyanates is also suggested.

 
Eye and face protection may also need to be considered for on the job protection as isocyanates are known to be an irritant to the eyes. 


Commonly Asked Questions

Q.   Can I wear an air-purifying, cartridge-style respirator for protection against isocyanates?
A.   Yes. Air-purifying respirators can be used with contaminants that have poor warning properties, such as isocyanates. However, you must check with the manufacturer of your respirator for specifics. For example, 3M and MSA have a respirator change-out formula for some isocyanates. If you know the airborne concentration in ppm, you can calculate about how long a cartridge will last. Please refer to Quick Tips #196: Respirator Cartridge Change Schedule for information.
 
Q.   In what types of products are isocyanates commonly found? What types of occupations involve exposure to isocyanates?
A.   Isocyanates are commonly found in paints, furniture cushions, mattresses and adhesives. Some professionals who might encounter isocyanates are auto-body repair-shop workers, insulation manufacturers, plastic wire coating and pipe manufacturers, truck and automobile painters, and tire and furniture manufacturers.
 
Q.   Are isocyanates considered a fire hazard? What type of fire extinguisher should be use to put out a fire that contains isocyanates?
A.   Classified as a Class III with a flash point of 527°F, isocyanates are not considered a serious fire hazard. Fires may be extinguished with carbon dioxide or dry-chemical extinguishers. However, when isocyanates burn, they do become harmful to anyone who is not equipped with a supplied-air respirator.

 

Sources

3M Job Health Highlights-Respirator Selection for Diisocyanates, Vol 18, August, 2009

American Journal of Industrial Medicine 13:331-349 (1988) "Isocyanates and Respiratory Disease Current Status"

Clinical Allergy. 1984, Volume 14, p.329-339.

(Rev. 9/2013)

 

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Please Note:
The content in this newsletter is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific compliance questions should refer to the cited regulation or consult with an attorney.

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