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Engineering Controls Database

Preventing Asthma and Death from MDI Exposure During Spray-on Truck Bed Liner Applications

Isocyanates are the leading attributable chemical cause of occupational asthma in the United States and many other industrialized countries [Tarlo et al. 1997]. The most widely used isocyanates are diisocyanates. Diisocyanates are a group of low-molecular-weight aromatic and aliphatic compounds that contain two isocyanates groups. The most common of these are toluene diisocyanate (TDI), methylenebis(phenyl isocyanate) (MDI), and hexamethylene diisocyanate (HDI). They are widely used in the manufacture of flexible and rigid foams, fibers, coatings such as paints and varnishes, and elastomers. Diisocyanates are increasingly used in the automobile industry, autobody repair, and building insulation materials. Spray-on polyurethane products containing isocyanates have been developed for a wide range of retail, commercial, and industrial uses to protect cement, wood, fiberglass, steel and aluminum, including protective coatings for trailers, boats, foundations, and decks, and truck beds.

It has been estimated that there are more than 3,000 spray-on truck bed-lining facilities in the United States, most of which are small businesses with fewer than five employees. These facilities are sometimes operated without the benefit of environmental and engineering controls to protect the sprayer from the adverse health effects of exposure to aerosolized MDI. Conservative estimates indicate that more than 10,000 workers are employed in the spray-on bed lining industry nationwide. Press releases from two of the major truck bed liner companies indicate that the spray-on bed liner industry is rapidly growing.
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; exposure may also occur through skin contact during the handling of liquid isocyanates.

MDI and other isocyanates may irritate the mucous membranes of the eyes, upper and lower respiratory tracts, gastrointestinal tract, and skin [NIOSH 2006]. Eye tearing, nose and throat irritation, and cough may occur [Littorin et al. 2000]. Respiratory irritation may progress to chronic upper and lower respiratory symptoms, although symptoms of local irritation do not reliably indicate chronic respiratory conditions [Wang and Petsonk 2004]. Acute respiratory distress syndrome or reactive airways dysfunction syndrome may also result from short-term high exposures [Tarlo et al. 1997b; Banks 1998]. MDI exposure can sensitize workers, making them subject to severe asthma attacks if they are exposed again. Isocyanates may cause cancer in animals but the carcinogenic potential of MDI in humans is not known. However, data from 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 [NIOSH 1986].

Because the odor threshold for MDI is many times above the NIOSH 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. 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 1986].
NIOSH conducted surveys at six spray-on truck bed liner sites.

Spray on truck bed lining process

The process involves applying a protective polyurethane or polyurea coating to the bed of pickup trucks or other vehicles and surfaces in a manner similar to undercoating. Bed liners are applied as a two-part resin. Part A is an MDI-based product; part B is usually a polyol or polyamine that reacts with the isocyanate to form a tough, resilient elastomeric surface coating. The MDI-based liners are applied to pickup truck beds and other surfaces to protect them from damage and to provide a nonskid surface. The spray-on application process is usually done in a spray enclosure (see Figure 1). The spray-on truck bed lining process is commonly performed in spray-on truck bed specialty shops, auto body centers, and auto dealerships.
Figure 1. Spraying base coat with protective equipment.

Figure 1. Spraying base coat with protective equipment.

NIOSH identified two spray-on processes commonly found in the industry [Heitbrink and Almaguer 2003]: one process applies truck bed liners at room temperature and pressures of approximately 50 pounds per square inch (low temperature/low pressure); a second process applies truck bed liners at temperatures above 165 °F and pressures of approximately 1,500 pounds per square inch (high temperature/high pressure). In both processes, the A and B components are pumped separately to the spray gun and mixed at the time of application. The worker uses a hand-held spray gun to apply the rapidly curing product onto the truck bed interior. The shape of the spray pattern is determined by the nozzle shape. To create a textured, nonslip surface on the bed, a small quantity of the coating is sprayed into the air, allowing the product to settle onto the truck bed. To obtain a bed liner thickness of 0.125–0.25 inches (a standard in the industry), approximately 50 pounds of the two-part resin (part A and part B) are sprayed onto the bed and inside walls of a typical pickup truck (see Figure 2).
Figure 2. Spraying texturizing coat with protective equipment.

Figure 2. Spraying texturizing coat with protective equipment.

A third type of spray-on process that was not included in the NIOSH sampling surveys is the cold-batch, pre-mix process. The information contained in this document may apply to this process as well. During this process, isocyanate/resin and catalyst materials are generally mixed in small quantities (quarts or gallons) at room temperature with a high-speed drill. Afterwards, the mixture is poured into a hopper gun and applied at 30–60 pounds per square inch from inside the truck bed. The chemicals used in the cold-batch operations typically contain less MDI (less than 20%). However, the cold-batch materials contain higher amounts of flammable solvents (toluene and N-butyl acetate) which should have their own exposure prevention and ventilation requirements.


Manufacturers and distributors of spray-on polyurethane/polyurea products containing MDI and other isocyanates should work together to assess and determine the best controls for the spray-on bed liner process. The ultimate goal is to establish procedures and develop controls (such as proper ventilation) to minimize MDI concentrations within the spray enclosures. These procedures and controls—together with respirator use, a written respiratory protection program, and ongoing worker training—will reduce the risk for worker exposures during the spray-on process.

General Responsibilities for Shop Owners and Workers to Prevent MDI Exposures

• Use any training, information, and literature available through the manufacturers, distributors, franchisers, and government agencies.
• Isolate the spray process by building an enclosure equipped with exhaust ventilation.
• Use a pressure manometer or tracer smoke along cracks and walls to ensure that the spray enclosure is under negative pressure and prevents the escape of MDI to adjacent work areas.
• Use a full-facepiece, supplied-air respirator and wear personal protective clothing such as hooded coveralls, chemical- resistant gloves, and footwear to prevent dermal absorption during the spray application process (see Figure 3).
• When entering or re-entering the enclosure immediately after spraying, use a full-facepiece, air-purifying respirator equipped with a combination N95 filter/organic vapor cartridge as minimum protection and wear appropriate clothing to prevent dermal exposure.
• To prevent fouling of the exhaust system and fan blades, filter all exhaust air at its collection point within the spray enclosure before it enters the exhaust system.
• Discharge exhaust from the spray enclosure away from occupied areas and ensure that the exhaust outlet is located away from HVAC air supply intakes and supplied-air respirator pumps/compressors.
• Ensure that only authorized persons trained in the use of safe work practices, ventilation, and personal protective equipment are allowed to perform spraying.
Figure 3. Worker wearing protective clothing and equipment

Figure 3. Worker wearing protective clothing and equipment

General Responsibilities of Manufacturers, Distributors, and Franchisers of Spray-on Chemicals and Equipment

Provide safety and health information to users of your chemicals and equipment. Include information about design of spray enclosures, engineering controls, safe work practices, and the hazards of exposure. These two references are excellent examples of useful information:
• MDI and TDI: Safety, Health and the Environment [Allport et al. 2003], by the International Isocyanate Institute Inc. for occupational safety and health information about MDI.
• The API brochure Truck Bed Liners: Worker Protection, available in both English and Spanish. The brochure outlines safety precautions for spray gun users who apply spray-on truck bed liners [API 2004].
• Develop, publish, and distribute additional safety and health resources to help workers using spray-on MDI products.

Product Substitution

When feasible, substitute a less hazardous material for MDI and other isocyanates. NIOSH policy is always to recommend using a less toxic substitute if available. Water-borne acrylic coatings, polysulfide rubber coating, and epoxy are other chemicals that have been used to make truck bed liners. Drop-in truck bed liners are also commercially available. NIOSH cannot address the quality or performance and has not addressed the safety of these products.
In addition, using substitute chemical processes or products may have advantages and disadvantages, including safety and health issues that are not addressed in this document.

Equipment/Formulation Modification

• Manufacturing and distribution: Investigate design changes that result in less aerosolization or fewer fugitive emissions during the spray process.
• Bed liner industry: Investigate potential process changes and work practices to determine whether reduced application pressure and slower application techniques result in lower MDI monomer air concentrations during the spray process.
• Chemical manufacturing: Consider changes in chemistry and formulation that would reduce the amount of fugitive MDI released during spraying.
These design, process, and formulation changes could reduce airborne MDI, resulting in reduced worker exposures. They would also place less financial burden on retail owners and operators in controlling airborne contaminants during the spray process. Furthermore, less overspray would produce less waste and provide further economic benefit for the retail owner. Unverified evidence from a spray-on bed liner company suggests that using lower pressures (10 pounds per square inch) may reduce airborne MDI concentrations and thus lower the exposure.

Spray Enclosure and Ventilation

• Design and build a spray enclosure to isolate the spray process and contain airborne MDI within the enclosure.
• Minimize the size of enclosure to the smallest floor area and room volume compatible with the operation. This helps to maximize the efficiency of the ventilation system.
• Equip the spray enclosure with an exhaust ventilation system to capture vapor and particulate MDI near the point of generation.
• Ensure that the spray enclosure is under negative pressure to prevent leakage of airborne MDI into other areas within the shop.
• Provide clean “make-up” air to the spray enclosure to replenish exhausted contaminated air.
• Place the make-up air supply and exhaust locations of this ventilation system so that the system generates a directed air (push-pull type) flow that maximizes ventilation efficiency and contaminant control.
• Discharge the exhaust high above the roof and away from air intakes, garage doors, or other openings where the exhaust could re-enter the shop. Also ensure that the exhaust is discharged above the roof recirculation region (See Industrial Ventilation: A Manual of Recommended Practice [ACGIH 2004] to determine the recirculation region).
• Control access to exhaust discharge areas by physical barriers and warning signs.

Work practices

Spray gun users should be trained to make their work practices compatible with the control concepts of the ventilation system:
• Avoid standing between the spray nozzle and the ventilation system exhaust hood.
• Work with the spray nozzle downstream of the breathing zone, where the MDI contaminant will be diluted, directed away from the breathing zone, and less likely to contribute to the worker’s exposure.
• Allow the exhaust ventilation system to operate for an additional period at the end of spraying operations before deactivating the exhaust or allowing personnel to enter the spray enclosure without proper respiratory protection. This delay allows the ventilation system to purge or remove most remaining airborne contaminants.
• Do not enter exhaust discharge areas without respiratory protection.

Ventilation system capabilities

• Obtain a site-specific exposure assessment that includes sampling data at desired intervals following the spray operation. These data are the best way to determine the purge time (time required to reduce MDI concentrations consistently below the NIOSH REL) required for your ventilation system.
• In the absence of exposure assessment data, determine the time required for the ventilation system to exhaust an air volume equal to a single room air change.
• Calculate the required purge time, allowing sufficient time for at least three complete air exchanges—or even longer for systems with poor air mixing.
• During the purge time, use respiratory protection when entering the spray enclosure.
• For assistance calculating the time required for a single air change and the purge time of the ventilation system, see Appendix B [NIOSH 2006].

Ventilation maintenance

• At least once per month, inspect and maintain the ventilation equipment to ensure adequate system performance (or more frequently if a decrease in performance is suspected).
• Document and re-evaluate baseline ventilation performance measurements, including hood static pressure, pressure drop across filters, and velocity or volumetric flow measurements for indications of system performance degradation.
• Develop a maintenance schedule to change exhaust filters as needed to prevent reduced airflow.
• Ensure that exhaust fans have inspection access doors.
• Ensure proper filter placement at air exhaust points to prevent filter bypass and the fouling of the exhaust system.
• Inspect fan blades regularly for material buildup and clean them to prevent a decrease in system performance.
• Check fan belts for deterioration and slippage and replace as necessary.
• Follow lock-out, tag-out procedures [29 CFR 1910.147] during inspection and maintenance procedures whenever worker injury may result from unexpected start-up of this equipment.

Spraying in Enclosed Areas

• Take additional precautions when spraying inside vans, enclosed trailers (e.g., horse trailers), and other similarly enclosed spaces.
• Determine whether the area to be sprayed meets the definition of a confined space and, if so, follow all OSHA requirements for working in a confined space [29 CFR 1910.146].
• Use supplemental local exhaust ventilation in addition to the exhaust ventilation of the spray enclosure.
• Wear a full-facepiece, supplied-air respirator and appropriate clothing to minimize the exposure.

Worker Isolation

• Restrict access to the spray enclosure to spray gun users.
• Allow only essential workers wearing appropriate respiratory protection into the truck preparation areas or other areas where workers may be exposed to isocyanates.

Exposure Monitoring

• If you are a shop owner, conduct an exposure assessment for airborne MDI at the onset of business and any time a major change (structural or ventilation) is made to the spray enclosure.
o Even if you install a ventilated enclosure design tested as effective for a product and process, conduct exposure monitoring to verify the effectiveness and proper installation at that site.
o Ensure that spray gun users and the areas adjacent to the spray enclosures are sampled.
o Use the appropriate sampling methods, as described in the NIOSH and OSHA analytical methods.
• Use the NIOSH REL of 0.2 mg/m3 as a 10-minute or the OSHA PEL of 0.2 mg/m3 as a 15-minute ceiling limit concentration when interpreting exposure monitoring results. The short-term, intermittent nature of the spray-on truck bed liner process makes the NIOSH or OSHA ceiling limits the most appropriate exposure criteria for this process. Collect personal breathing zone samples from the time a spray operation begins to the time when spraying has stopped (typically a 10–20 minute period). Compare results with the NIOSH or OSHA ceiling limit.

Detailed recommendations for respiratory protection, personal protective clothing, worker and employee education, medical monitoring, surveillance and disease reporting, and decontamination and waste disposal can be found in the NIOSH Alert [NIOSH 2006].

NOTE: The control information in this engineering control write-up is taken primarily from the following publication:
NIOSH [2006]. NIOSH Alert: preventing asthma and death from MDI exposure during spray-on truck liner and related applications. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2006-149.
ACGIH [2004]. Industrial ventilation: a manual of recommended practice. 25th ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, Committee on Industrial Ventilation.

Allport DC, Gilbert DS, Outterside SM [2003]. MDI and TDI safety, health, and the environment. Chichester, West Sussex, England: John Wiley and Sons, Ltd.

API [2004]. Truck bed liners : worker protection. Arlington, VA : Alliance for Polyurethanes Industry, Publication No. AX–362.

Banks DE [1998]. Respiratory effects of isocyanates. In: Rom WN, ed. Environmental and occupational medicine 3rd ed. Philadelphia, PA: Lippincott-Raven Publishers, pp. 537–563.

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office of the Federal Register.

Heitbrink WA, Almaguer D [2003]. Recommendations for the study of control measures for overspray generated during bed liner application. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Report No. EPHB 294–11a.

Littorin M, Rylander L, Skarping G, Dalene M, Welinder H, Strömberg U, Skerfving S [2000]. Exposure biomarkers and risk from gluing and heating of polyurethane: a cross sectional study of respiratory symptoms. Occup Environ Med 57:396–405.

NIOSH [1986]. Current Intelligence Bulletin No. 47: 4,4’-Methylenedianiline (MDA) (revised). Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 86–115.

NIOSH [2006]. NIOSH Alert: preventing asthma and death from MDI exposure during spray-on truck liner and related applications. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2006-149

Tarlo SM, Liss GM, Dias C, Banks DE [1997]. Assessment of the relationship between isocyanate exposure levels and occupational asthma. Am J Ind Med 32(5):517–521.

Wang ML, Petsonk EL [2004]. Symptom onset in the first two years of employment at a wood products plant using diisocyanates: some observations relevant to occupational medical screening. Am J Ind Med 46:226–233.
and auto dealerships
auto body centers
spray gun users
spray-on truck bed specialty shops
truck bed sprayers