In general, the “toxicity” of a cleaning fluid is measured through the use of 8-hour “time-weighted average exposure limits” (AEL). Similar terms you might find on labels and SDS sheets include “Average Exposure Limits” (AEL), “Personal Exposure Limits” (PEL) and “Threshold Exposure Limits” (TEL). One special term is “Threshold Limit Values” (TLVs) which only are established by the American Conference of Governmental Industrial Hygienists (ACGIH).

It is prudent to understand the underlying basis for exposure limits recommended or mandated by an organization or vendor. Some exposure limits are set simply to prevent acute effects such as eye or respiratory tract irritation that could make working uncomfortable or difficult to perform. Other exposure limits are set to prevent more serious immediate effects such as death by asphyxia (lack of oxygen). Most exposure limits are set to prevent long-term damage to organs of the body that could occur if high exposures continued over a period of time (days, weeks or even years). There are also exposure limits for shorter time periods and even “instantaneous” exposure limits which further quantify the risk of using a specific chemical.

These consensus values are considered protective of most (but not necessarily all) workers that would be exposed at the specified concentration for up to 8 hours per day, 40 hours per week, for a working life time (which is taken as 40 years). These ratings often are described as the “8-hour time-weighted exposure limit.”

In practical terms, a PEL defines the maximum safe concentration of a vapor in the air to which an employee could be safely exposed over the course of the workday. Note there could be periods of time spent working above the exposure limit as long as the average was below the PEL (that’s why it’s called a time-weighted average). Those periods would be offset by periods of time spent below the exposure limit. The result would be, that by the end of the workday, the average exposure would be 10 ppm or less.

There are other types of TLVs and PELs. For example, there are short-term exposure limit which are generally higher than the 8-hour TWA, there are acceptable daily limits, instantaneous exposure limits which must never be exceeded, and no-observed-adverse-effect levels. But the generally accepted standard for worker safety is the 8-hour TWA TLV.

One other bit of background: the “8-hour time-weighted exposure limit” is estimated to be one-tenth the minimum airborne concentration of vapors to which workers may be repeatedly exposed for an eight-hour workday over a hypothetical 40-year career without suffering any possible health effects. So a chemical with a 200 ppm TLV would typically have a 2,000 ppm no-observed-adverse-effect rating. All exposure limits are expressed in “parts per million” (ppm). For example, about 18% of the air we breathe is pure oxygen. This means that oxygen is present in the atmosphere at 180,000 parts per million.


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