Thursday, January 06, 2011

A minor correction on a previous tear gas posting

In my update to my posting about Physicians for Human Rights and tear gas, I wrote:

Some blogs are saying "The U.S. Army Center for Health Promotion and Preventive Medicine warns that at a concentration of 2mg per cubic meter, CS gas 'is immediately dangerous to life [and health]…'

The document does say that - and it is a typo.
It turns out it was not a typo - but it doesn't mean what it sounds like.

The phrase "immediately dangerous to life and health," IDLH, is a specific technical term that is defined by OSHA as "an atmosphere that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair an individual's ability to escape from a dangerous atmosphere." The word "impair" is the key word here, meaning it makes it more difficult to leave but does not prevent the individual from doing so.

For those keeping score, there are three important acronyms when dealing with tear gas: IDLH, as defined above, ICt50, which is the concentration that is intolerable to 50% of the exposed population for 1 minute, and LCt50, which is the concentration that is lethal to 50% of the exposed population for one minute.

For CS tear gas, the numbers are

Odor threshold value - 0.004 mg/m3 (concentration that can be smelled)
IDLH - 2 mg/m3
ICt50 - 3.6 mg/m3 (depending on which study; some humans can learn to tolerate up to 6.6 mg/m3)
LCt50 - 61,000 mg/m3

So while it seems counterintuitive that IDLH would be lower than ICt50, the OSHA definition makes it make sense.

The huge difference between ICt50 and LCt50 is what makes CS such a perfect tool for crowd control. The amount needed to get someone to run away is far, far less than the amount necessary to kill someone.