Preventing Asthma and Death from MDI Exposure

         Spray-on polyurethane/polyurea products containing isocyanates such as MDI have been developed for a wide range of retail, commercial, and industrial uses to protect cement, wood, fiberglass, steel, and aluminum surfaces such as truck beds, trailers, boats, foundations, and decks.  MDI is methylenebis(phenylisocyanate).

         In this fact sheet, MDI refers to all MDI-based isocyanates.  MDI, toluene diisocyanate (TDI), and the polyisocyanate products based on the diisocyanate hexamethylene diisocyanate (HDI) are the most commonly used diisocyanates in the polyurethane industry.  Isocyanates are widely used in the manufacture of flexible and rigid foams, fibers, coatings such as paints and varnishes, and elastomers.  Isocyanates are increasingly used in the automobile industry, autobody repair, and building insulation materials.

          Production and consumption data for the polyurethane industry in the United States and North America are available through the Alliance for the Polyurethanes Industry (API). Their publications End-Use Market Survey on the Polyurethane Industry [API 2003] andSocio-Economic Impact of Polyurethanes in the United States [API 2004a] provide the following facts:

  • In 2002, the U.S. market for polyurethane was 5,528 million pounds (87% of the North American market).
  • The manufacture of products such as flexible slabstock, flexible molded foams, rigid foams, coatings, binders, elastomers, adhesives, sealants, reaction injection molding, thermoplastic polyurethanes, and spandex used 212 million pounds of MDI as a raw material and 1.3 billion pounds of polymeric MDI in 2002.
  • About 253,700 workers at 853 locations throughout the United States are employed in transportation industries that use polyurethane. Uses include resistant polyurethane coatings such as truck bed liners and other products such as headrests, armrests, adhesives, insulation, and other molded components used in vehicle production.

          Isocyanates are the leading attributable chemical cause of occupational asthma in the United States and many other industrialized countries.  Workers with asthma symptoms from isocyanate exposure often continue to have symptoms after exposures have been terminated.  Affected workers often have to leave their jobs to prevent progression of respiratory symptoms.

          The major route of work-related exposure to MDI is inhalation of the vapor or aerosol.  Because the odor threshold for MDI is many times above the recommended exposure limit (REL), smell should never be relied on as an indication of exposure, nor should the absence of odor be used to indicate safety.  MDI can be detected by odor only after dangerous concentrations exist, resulting in potential overexposure.

          Exposure may also occur through skin contact during the handling of liquid MDI-based products. Work-related exposure normally occurs during the spray application of MDI-based products.  In 1996, NIOSH issued Preventing Asthma and Death from Isocyanate Exposure, which summarizes reported cases of disease and death following occupational exposure to diisocyanates and diisocyanate-based products.

Irritation and Lung Injury

          MDI and other isocyanates may irritate the mucous membranes of the eyes, upper and lower respiratory tracts, gastrointestinal tract, and skin.  Eye tearing, nose and throat irritation, and cough may occur.  Respiratory irritation may progress to chronic upper and lower respiratory symptoms, although symptoms of local irritation do not reliably indicate chronic respiratory conditions.  Acute respiratory distress syndrome or reactive airways dysfunction syndrome may also result from short-term high exposures.

Respiratory Sensitization

          Isocyanates can sensitize workers, making them subject to severe asthma attacks if they are exposed again, even when concentrations are continuously below the NIOSH REL.  Skin exposures may be associated with the onset of respiratory symptoms.  Respiratory disorders associated with isocyanate exposure include asthma and hypersensitivity pneumonitis.  Sensitization may result from a single episode of overexposure or intermittent exposures at low concentrations.  Once a worker is sensitized, even low concentrations may trigger symptoms such as wheezing, chest tightness, shortness of breath, and cough.  Persons with chronic hypersensitivity pneumonitis may also experience fatigue and weight loss.  These symptoms may begin immediately or may be delayed for up to 8 hours after exposure. Death from severe asthma in sensitized subjects has been reported.


          Isocyanates may cause cancer in animals; however, evidence is insufficient to describe the carcinogenic potential of MDI in humans.  Data from recent studies show that methylene dianiline (MDA), a known animal carcinogen and the principal metabolite of MDI monomer, is found in the blood of MDI-exposed rats and in the urine of humans exposed to a mixture of polymeric MDI and MDI monomer.  Another study found that a commercial grade of MDI (45% MDI monomer by weight) induced chromosome aberrations in human blood lymphocyte cultures after a 24-hour treatment.  NIOSH recommends that work-related exposure to MDI be minimized because of the potential for respiratory sensitization and the potential carcinogenicity of the metabolite MDA.


          NIOSH recommends that MDI monomer exposure be limited to 0.05 milligram per cubic meter of air (mg/m3) or 0.005 part per million parts of air (0.005 ppm) as a time-weighted average (TWA) for up to a 10-hour workday during a 40-hour workweek, with a ceiling limit of 0.2 mg/m3 (0.02 ppm) for any 10-minute period.  This NIOSH REL is intended to prevent acute and chronic irritation and sensitization of workers but not to prevent health effects in workers who are already sensitized.  Available data do not indicate a concentration at which MDI fails to produce adverse reactions in sensitized persons.  Unless otherwise stated, use of the term NIOSH REL in this document means the NIOSH REL of 0.2 mg/m3 as a 10-minute ceiling concentration when referring to spray-on truck bed liner processes.


          The current Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for MDI monomer is 0.2 mg/m3 as a ceiling limit (0.02 ppm) [29 CFR* 1910.1000 (a)(1)].

Workers should take the following steps to protect themselves from MDI* exposure during spray-on applications:

1.   Be Informed.

  • Participate in medical exams, medical monitoring, air monitoring, respiratory fit-testing, and training programs offered by your employer.
  • Know the safety and health information and follow all safety precautions provided by the manufacturers about the product(s) you are spraying.

2.   Use respirators and PPE safely.

  • Make sure that you have been fit-tested for your respirators and that you have been trained in their proper use, storage, and maintenance.
  • Regularly inspect and maintain all component parts of supplied-air respirators and air-purifying respirators to assure their continued effectiveness.  The components include aircompressors, hoses, regulators, facepiece, etc.
  • When spraying MDI,
    a)  always use a full-face piece, supplied-air respirator operated in a pressure demand or other positive-pressure mode and
    b)  wear appropriate personal protective equipment (PPE) (such as hooded coveralls, chemical resistant gloves, and footwear).
  • Never remove or lift the respirator away from the face while in the spray enclosure. Use clear plastic tear away sheets on the visor to provide better visibility throughout the spray process.
  • Be aware that the highest MDI concentrations occur inside the spray enclosure.
  • After spraying, leave the spray enclosure before removing your respirator and PPE and keep the ventilation system running to exhaust any remaining MDI.
  • Do not re-enter the spray enclosure after spraying has stopped without PPE and a respirator (minimum protection is an air-purifying respirator with a full facepiece equipped with a combination organic vapor/
    N95 filter cartridge).
  • Know the change-out schedule for the air purifying respirator cartridge and strictly follow the schedule to make sure that you are adequately protected from exposure to MDI.

3.  Check the ventilation system in the spray enclosure.

  • Make sure that the spray enclosure ventilation is operating correctly before beginning the spray process.
  • Ensure ventilation system intake filters are not clogged and are correctly installed in their frames.
  • Keep the ventilation system running until the MDI aerosol has been cleared from the spray enclosure.

4.  Use good work practices and personal hygiene.

  • Never eat or drink in work areas.
  • Wash hands and face before eating, drinking, or smoking.

5.  Report symptoms.

a)  Do the following if you have symptoms:

  • Tell your employer if you develop breathing problems that could be related to MDI exposure (wheezing, chest tightness, shortness of breath, or coughing).
  • Seek medical evaluation of those symptoms.
  • Do not spray or remain exposed to MDI until you are medically evaluated for breathing problems that might be related to the spraying process.

b)  If you have been diagnosed with occupational asthma due to sensitization from MDI, do not work where you might be exposed to MDI.

Employers should take the following steps to protect workers from MDI exposure during spray-on applications:

1.  Provide workers who may be exposed to MDI with information and training about MDI as required by the OSHA hazard communication standard [29 CFR† 1910.1200].

  • Inform workers about the serious health effects that may result from exposure to MDI and provide them with safety and health information.
  • Inform workers about any materials that may contain or be contaminated with MDI.2. Provide respiratory protection and personal protective equipment to workers who may be exposed to MDI.

2.  Provide respiratory protection and personal protective equipment to workers who may be exposed to MDI.

  • Develop, implement, and enforce a worksite-specific respiratory protection program that provides for medical monitoring, medical examinations, respirator fit-testing, and respirator training. †Code of Federal Regulations.
  • Provide appropriate PPE (hooded coveralls, chemical resistant gloves, and footwear) to workers who use spray guns and to other workers who may be exposed to MDI.
  • Train workers to properly use, store, and maintain their respirators.
  • Regularly inspect and maintain all component parts of supplied-air and air-purifying respirators to assure their continued effectiveness.  The components include air compressors, hoses, regulators, facepieces, etc.
  • Make sure respirators are professionally fitted.
  • When workers must wear prescription eye glasses under a full-facepiece respirator, provide them with prescription inserts designed to be compatible with the respirator.

Supplied-air respirators

          When workers are spraying MDI or are inside the spray enclosure during spraying, make sure they use full-facepiece, supplied-air respirators operated in a pressure-demand or other positive-pressure mode.

          Make sure that supply air for the supplied-air respirator is taken from a clean, uncontaminated area that is well removed from any aerosolized MDI or other contaminants.

Air-purifying respirators

          When workers must enter or re-enter the spray enclosure after spraying, make sure they use full-facepiece, air-purifying respirators equipped with a combination organic vapor/N95 filter cartridge.  This is minimum acceptable protection for these conditions.

          Establish and implement a change-out schedule for air-purifying respirator cartridges.

Provide engineering controls.

          Build a spray enclosure equipped with an exhaust ventilation system to isolate the spray process from the rest of the facility and maintain the enclosure under negative pressure to control and contain MDI aerosols in the spray enclosure.

          Determine the number of air changes per hour provided by the ventilation system and use this information to calculate the time required (after spraying) to reduce airborne MDI concentrations below the NIOSH recommended exposure limit (REL).

          Make sure that workers are trained to use engineering controls (such as spray enclosures with effective ventilation) and work practices to minimize MDI exposures.

          Allow only trained workers wearing NIOSH-approved, full-facepiece, supplied-air respirators to enter the spray enclosure during spraying.

          Establish and implement a preventive maintenance program for the ventilation system.

Provide medical examinations and surveillance for workers potentially exposed to MDI.

          Provide a preplacement medical examination and periodic medical monitoring for all potentially exposed workers to detect and prevent the acute and chronic effects of MDI exposure.

          Remove from the workplace any worker showing signs or symptoms of MDI exposure.

          Make sure the worker is medically evaluated before he or she is allowed to return to work.

          If the medical evaluation determines that the worker is sensitized, the worker must not be allowed to return to a job where MDI is used.

This fact sheet was based on information from NIOSH.  For a copy of the NIOSH Fact Sheet and more information on exposures to MDI go to