Page 26 - OHS, May 2020

INDUSTRIAL HYGIENE
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.
BY DAN CORCORAN
22 Occupational Health & Safety | MAY 2020
www.ohsonline.com
Airborne dusts are common to many work- places, many of which pose significant risks to workers due to their toxicities, sizes and configurations of their particles. As a result, OSHA has developed standards and Permissible Ex- posure Limits (PELs) to control workplace airborne exposures. Not all particulates pose as severe of a uni- versal threat as those mentioned above; however, they may cause an allergic reaction resulting from inhala- tion. We will subsequently take some time to consider particulate matter inhalation issues with specific con- sideration 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 air- borne 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 sub- stance 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 “par- ticulates not otherwise regulated” (PNOR). The per- missible 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 re- spirable 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 fil- ter 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 re- spirable 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 mi- crometers in diameter and have different aerodynam- ic 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 par- ticles 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.
Once particles find their way into the alveolar ar- eas of the lungs, the final disposition varies. For in- stance, 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
Hypersensitivity pneumonitis occurs when people breathe in certain chemicals or particulate matter that triggers an immune response. In response, the indi- vidual’s immune system does not shut down prop- erly 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, bron- chitis, 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