The first day of my Ergonomics course at the University of Michigan Professor Strong explained how chairs, mostly not adjustable in those days, were built for a 6′ tall male, which explained a LOT. He went on to point out that 6′ people were not the majority at the time they were producing chairs only they would find comfortable. To this day, office workers and people who are shorter than 6’ tall spend a lot of time on non-adjustable seats and experience back and other ergonomic problems.
The Occupational Safety and Health Administration’s (OSHA’s) Permissible Exposure Limits (PELs) were adopted wholesale from the 1968 American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs). The data used to develop those limits often came from studies conducted by the military. As a result, they are protective for young, healthy, fit men. As time went on the ACGIH added new, or adjusted some, TLVs from research that includes both male and female volunteers made up of graduate students or students, still not representative of the majority of the work force. Some research is conducted on current worksites and some, where enough data is available, from tracing data backward from the onset of certain illnesses to their occupational exposures sometimes decades before.
To access the TLVs you usually have to be an ACGIH member or purchase them. However, OSHA created a webpage with a Table that compared multiple exposure limits including a link to each chemical’s TLV, it does not contain OSHA skin notations. Often your Safety Data Sheet (SDS) will include OSHA exposure limits, ACGIH TLVs, and some include the National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limits, or Occupational Exposure Limits (OELs) from other standards-setting organizations or governments. This is another reason you want the most recent version of the SDS.
Most exposure limits, including TLVs and PELs, have a 10X safety factor. This means they take the concentration at which the Lowest Observable Effects Level (LOEL) occurs and divide it by 10 to set the limit. However, even a 10X safe level, may not be protective when your employees are petite females, or over 50 years old, and/or are diabetic, pregnant, asthmatic, and so on. While carcinogens are given exposure limits the point at which an individual’s body cells may react with the carcinogen varies greatly, both above AND below the exposure limit. However, a limit must be set for carcinogens that are used in industry to protect as much of the occupational workforce as possible.
However, there are many variables to consider when assessing employee exposures. With processes where multiple chemicals have the same target organ, common with solvents, the exposure needs to be calculated using fractions made of the concentration of each chemical, divided by its exposure limit must be less than one or the employee is overexposed. The equation is:
There are variables introduced by employee’s off shift time. such as activities during time away from work. An employee with exposure to a light fluffy chemical that sticks to any surface, such as skin, reacts with the sun increasing the risk of skin cancer when, it turns out, the employee loves to bike marathons in his spare time. When I met him he had recently had a skin lesion removed from his face. Or, an employee works as a mechanic at night and often cleans grease off his skin with gasoline or acetone, creating a breach in a normally protective barrier that allows home and work chemicals into the body by an additional route. In this case, the air monitoring data is no longer representative and you have no idea what the body burden (amount of the chemical that is in the body) is at the end of the work shift without a Certified Industrial Hygienist.
There are the variables introduced by the employer. Very few of OSHA’s exposure limits require correction for extended shifts. For example, 1910.95 has an extended shift (greater than 8 hours) formula to reduce the Action Level based on the duration of exposure to noise due to the shortened recovery time for the nerves in the ear before the next shift begins. Speaking of noise, Ototoxic chemicals adversely affect hearing thresholds at noise levels below the noise standard’s exposure limits. But let’s leave that for the next blog.
