Questions about Vapor Exposure

There are a mess of questions out there when it comes to exposure to vapors in the air and quite a few of them orbit around whose standards apply and when they apply. Is this OSHA’s bailiwick? What the heck is the ACGIH? Is EPA in charge?

So, we wanted to answer your question of “Whose standard applies when (and where) and what the heck do all of those numbers mean?”

To come up with a semi-approachable answer, we took the indoor air results from a client’s operational dry cleaner and then looked into the OSHA regulations for occupational exposure to tetrachloroethylene (perc/PCE).

Here’s a brief walk-through in what the levels (from the Indiana Department of Environmental Management / US EPA and the US Occupational Safety and Health Administration, or OSHA) mean and how they come about:

OSHA’s Numbers

OSHA establishes Permissible Exposure Limits (PELs) based on an 8-hour day, 40-hour work week. They use a Time Weighted Average (TWA) of exposure to hazardous substances that they calculate after talking to the National Institute for Occupational Safety and Health (NIOSH), a division of the Centers for Disease Control (CDC).

These levels can be accessed at the OSHA PEL Table ( ).  These standards determine which levels are acceptable levels for an average worker’s exposure to hazardous substances.  Because this is an average over an 8-hour work day, the PEL also contains a “ceiling” limit to which a worker must not be exposed for more than 5 minutes in a 3-hour period.  Those are the official rules for occupational exposure to hazardous substances.

Beyond those rules, there is also guidance from the American Conference of Governmental Industrial Hygienists (ACGIH) called the Threshold Limit Value (TLV), which is not a rule, but a suggested limit, without the force of law, which tends to be stricter than those rules from OSHA but doesn’t carry the force of law.

The Environmental Regulators

IDEM and US EPA establish their own indoor air quality standards for residential and commercial exposure – this isn’t a worker’s exposure to a chemical that they expect to be exposed to through the course of their work, this is a person’s exposure to a hazardous substance that has been released into the environment and has triggered a response action from the environmental regulatory agency. It’s the difference between being exposed to gasoline fumes because you’re a mechanic and being exposed to gasoline fumes because they’re seeping into your house because the gas station next door has a ruptured tank.

These are default standards which are general guidelines, which means they’re extra conservative.  The sampling procedure that the IDEM has shown a preference for asks for an indoor air sample to be taken in the winter, when the heater is running (which creates a vacuum, pulling the vapors from the contaminated soil), the windows and doors are to be closed; the sampling location is 3 to five feet above the floor in an occupied area and the sample is collected over a 24-hour period in order to get the exposure level.

This is all well and good, but think about it: that means that the exposure measured by the stationary sampling container is the equivalent of a person sitting in his La-Z-Boy on the first floor of his house and not moving for 24-hours straight.  While I may have done that once or twice in college, it’s not something that people make a habit of and that is the exposure that is calculated.

To keep it in perspective: in the course of 24 hours, an average person does things like open and close his doors, move about his house, go to work and sleep in his bed on the second floor of his house at night.  None of that invalidates IDEM’s default levels, but it does put the conservative nature of the level into perspective.

Even a Belt and Suspenders Sounds Risky to These Guys

In Indiana, the IDEM sets the Commercial Indoor Air 1-year Action Level for perc at 51 micrograms per cubic meter, which translates to 7.52 parts per billion by volume (ppbv).  Neither of those are very ‘friendly’ numbers, so think of it this way: an Olympic sized swimming pool (50 meters X 16 meters X 2 meters) has, give or take a few thousand liters, 4,800,000 liters of water or 4,800,000,000 (4.8 billion) milliliters of water, to put it in “billions.”

For the concentration IDEM has set up for perc as a level requiring investigation and mitigation, you’d be looking at 36 milliliters of perc, which is just about the volume of a bottle of Visine dumped into an Olympic-sized pool.  The results we got from the indoor air sampling we did at our client’s drycleaner translate to 75.2 and 63.4 ppbv, which is above IDEM’s 1-year Action Level.


Safe, but not Crazy

OSHA’s PEL for perc is 100 parts per million by volume (ppmv) with a ceiling of 200 ppmv for no more than 5 minutes every three hours.