Airborne Dust Exposure and Hypersensitivity Pneumonitis

Airborne Dust Exposure and Hypersensitivity Pneumonitis

Not all airborne dust particles are the same. Here are ways you can differentiate between particulate exposures and hazards for your workplace. 

Airborne dusts are common to many workplaces, many of which pose significant risks to workers due to their toxicities, sizes and configurations of their particles. Various metal fumes, such as lead and cadmium, can be highly toxic if individuals are exposed to airborne concentrations found in some work environments. Particulates such as asbestos and crystalline silica are also quite hazardous and can cause severely debilitating diseases.

As a result, OSHA has developed standards and Permissible Exposure Limits (PELs) to control workplace airborne exposures. Not all particulates pose as severe of a universal threat as those mentioned above; however, they may cause an allergic reaction resulting from inhalation. We will subsequently take some time to consider particulate matter inhalation issues with specific consideration given to airborne particulates that might result in an allergic response.

Measuring Particulate Matter Air Contaminant Levels

One of OSHA’s primary tools for dealing with airborne dusts or particulate matter (PM) is to establish PELs for particulates that pose a specific health threat. These exposure levels may be indicated in OSHA’s Z-tables in 29 CFR 1910.1000, or they may be included in an expanded health standard for a given toxic substance that details specific steps employers must take to limit employee exposures.

Particulate exposures that do not have a specific PEL identified are covered in the Z-tables under “particulates not otherwise regulated” (PNOR). The permissible exposure limit for the PNOR non-respirable fraction is 15 milligrams/cubic meter based on an eight-hour time weighted average. The PEL for the respirable fraction is 10 milligrams/cubic meter.

In measuring particulate exposure, a sampling pump is attached to a worker during the entire work shift, which pulls air across a filter at a known rate in liters per minute. The amount accumulated on the filter is determined and compared to the total volume of air that passed across the filter. When measuring the respirable fraction, the contaminated air first flows through a device that selectively screens out larger particles, leaving only the smaller particles to deposit on the filter.

Particulate Size Matters

The reason OSHA’s permissible exposure limit for respirable dust is much lower than total dust is because the smaller particles that make it to the filter have a much smaller diameter than the particles that are sifted out of the air stream prior to filter deposition. These smaller particles are typically less than 10 micrometers in diameter and have different aerodynamic properties as compared to larger particles.

Particulate matter is bad news for a person’s lungs. When a person inhales, for instance, larger particles are likely to impinge on the mucous-coated walls of the bronchi and bronchioles. They are then carried out of the lungs by the ciliated epithelium that lines these portions of the respiratory system. These particles are essentially swept up against gravity into the area of the throat and are swallowed. Smaller particles tend to follow the air stream and are more likely to find themselves deep in the lungs in proximity to the alveolar air sacs where they may impinge on the air sac walls and cause significant problems.

Types of Particles in the Lungs

Once particles find their way into the alveolar areas of the lungs, the final disposition varies. For instance, if the particle is soluble, it may be absorbed into surrounding tissues. Particulate matter such as crystalline silica and asbestos are not soluble, and the immune system will try to rid the area of the foreign body. White blood cells will attempt to break down the particle, resulting in the death of the white blood cells and scarring or fibrosis in the area where the particles were deposited. However, not all types of particulates are as resilient as asbestos fibers and crystalline silica, and may be digested and removed by the activities of white blood cells.

Hypersensitivity Pneumonitis

Most well-known diseases caused by inhalation of airborne particulates are the result of toxic properties of the particulate. Diseases such as asbestosis, silicosis, and other well-known diseases due to inhalation of heavy metal exposure have resulted in the promotion of specific OSHA standards. There are, however, many airborne particle exposure scenarios that are much less common that may result in a severely debilitating ailment known as hypersensitivity pneumonitis. Although rare, this condition can be quite debilitating and can occur regardless of the amount of respirable fraction of the dust inhaled.

Hypersensitivity pneumonitis occurs when people breathe in certain chemicals or particulate matter that triggers an immune response. In response, the individual’s immune system does not shut down properly and the interstitial space where gaseous transfer between the alveoli and the circulatory system takes place becomes inflamed. Depending on the duration of exposure, signs and symptoms may be classified as acute, subacute or chronic. Acute symptoms may include flu-like symptoms, rales and cough. Subacute signs and symptoms include continuous cough, bronchitis, shortness of breath and even anorexia. Chronic bronchitis may include similar signs and symptoms but may also include fatigue, fibrosis of the lung and clubbing of the fingers.

Since hypersensitivity pneumonitis is rare, it is not unusual for individuals who are exhibiting signs and symptoms of the condition to be overlooked. OSHA standards are established to deal with exposure related issues that impact the larger population of workers in industry. Workers and employers might not make the connection between the workplace exposure and the employee’s condition. The employee might attribute the condition to something else such as asthma or more common allergens, such as pollen. Over time, failing to identify the actual source of the condition can lead to chronic manifestations of the disease including lung fibrosis. It is subsequently important that potential exposures are identified.

Recognition of Hypersensitivity Pneumonitis

In considering the difficulties faced by an individual who suffers from hypersensitivity pneumonitis, it is important for employers to do what they can to both recognize the possibility of exposure in the workplace and to help to mitigate such exposures. A first step in identifying the potential of exposure is to consider possible sources of allergens.

Generally speaking, hypersensitivity pneumonitis results from respirable particles that get into the alveolar regions of the lung that are of biological origin such as fungal spores, or plant and animal proteins. Some well-known examples that have been identified over the years include farmer’s lung from various mold and bacterial spores from moldy hay, humidifier lung from various bacterial and fungal spores that are abundant in stagnant water, laboratory worker’s lung originating from proteins in rodent urine, cheese worker’s lung, malt worker’s lung, wood worker’s lung and bird fancier’s lung.

In essence, if workers are working in an environment that involves biological materials that are broken down into fine particles that can be inhaled, or in situations where bacteria or mold is prevalent, hypersensitivity pneumonitis is a possibility.

Prevention of Hypersensitivity Pneumonitis

In order to prevent this condition, it is important to recognize the possibility of occurrence. This can be done by determining if employees are exposed to organic dusts when performing their jobs or if microbial habitat is present that might result in biological activity. Speaking to frontline workers could provide insights as well to determine if any signs or symptoms consistent with the condition are exhibited by any employees. Ideally, it would be best to have an industrial hygienist or other appropriately trained health professional involved in the evaluation process. This evaluation may include verbal employee screening, medical evaluations, and air contaminant sampling. Maintaining local ventilation systems to capture dust at the source can also be a big help in preventing exposure as can separating employees from potential exposures.


Airborne particulate matter in the workplace continues to pose a significant health risk for many occupations. OSHA has developed standards and permissible exposure levels for a number of high-risk particulates and has also established exposure levels for both the total and respirable fraction of non-specific airborne particulates. These exposure levels do not take into account unique situations such as airborne particulates that may be prevalent in various workplaces that can cause an allergic response in specific individuals. This is a particular issue when such particulates are of biological origin such as fungal or bacterial spores or dusts generated from abrasion.

Individuals exposed to such particulates may exhibit signs and symptoms consistent with what has come to be known as hypersensitivity pneumonitis. It is important, therefore, that safety practitioners be cognizant of this possibility, particularly if microbial habitat is present in the workplace or if microorganisms are used in work processes (e.g. brewer’s yeast). Efforts should be made to evaluate worker health and prevent opportunities for exposure.

This article originally appeared in the May 2020 issue of Occupational Health & Safety.

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