“You need to take a closer look at real data, e.g., from the gas dispersion modelers.And I'd appreciate if you would highlight for your readers the Chlorine Institute's venerable Pamphlet 74, available for download free from their website, especially the pages on the 90-ton chlorine tank car.”Pamphlet 74
The Pamphlet 74 that Fred refers to is a very detailed discussion on how to determine the area at risk in the event of a variety of chlorine releases. Fred is correct that the Chlorine Institute offers this document as a free download. Unfortunately, they will not get any points for having a customer friendly web site to get the download; you have to go through a lot of steps to download a .PDF document. Be persistent, it is a valuable documents for anyone interested in discussing the very real threats from chlorine gas releases.
I re-read the discussion on modeling chlorine gas release scenarios this morning and I will have to admit that my description of the dispersion pattern is a tad bit simplistic. I wrote: “The wind disperses a chemical cloud in a fan shaped pattern.” In the early stages of a catastrophic release of chlorine there are a number of factors that cause some initial dispersion that is not wind dependant. This is the reason that the base of the dispersion pattern shown on page 24 of Pamphlet 74 is so wide.
To be fair, I also exaggerated the width of the cloud at its maximum extent by using the fan description. The width of the exposed area drops off fairly quickly at the far end of the dispersion pattern. Even so, the basic comment about the limited area of coverage stands. The area under the 3 ppm exposure limit on the diagram is a flattened oval 41.5 miles long and 2.3 miles wide, not a circular area with a radius of 41.5 miles.
While the entire circle is ‘potentially’ at risk from a chlorine release, only a small fraction (1.8%) would actually be exposed in the event of the actual release. The two charts on the next page of the pamphlet provide some additional important details about the dispersion of the toxic cloud. The first chart, Peak Concentration as a Function of Time, shows how long it takes for the toxic cloud to reach various distances. More importantly it shows that the cloud moves through an area, leaving the area with minimal continued exposure after it has move on.
This does ignore the fact that some residual chlorine gas could remain in low lying areas out of sunlight for significant lengths of time. This means that there will only be a limited need for decontamination after the incident. The second chart, Peak Concentration as a Function of Distance, shows how quickly the concentration drops off as the cloud disperses. To understand the practical affects of that change in concentration we need to understand the medical affects of chlorine gas as a function of exposure concentration.
Chlorine Exposure Effects
According to the OSHA web site for chlorine, an exposure “to 15 ppm causes throat irritation, exposures to 50 ppm are dangerous, and exposures to 1000 ppm can be fatal, even if exposure is brief”. As with any chemical exposure the longer one is exposed to a given concentration of the chemical, the greater the potential harm. Thus we can see that the peak concentration drops below irritant level fairly quickly.
Chlorine is Very Dangerous
Now, having exposed the hype, it must be clearly understood that there will be a significant area under the exposure curve where people will die if they are not adequately protected against exposure. There is an even larger area where there will be serious medical consequences from exposure to the peak concentration levels as the toxic cloud passes through the area. Looking at the charts on pages 24 and 25 of Pamphlet 74 it looks like anyone inadequately protected in the cloud for up to a couple of miles away from the catastrophic release from a full railcar is at serious risk of being killed by the cloud. Inadequate protection in the cloud at distances of up to 15 miles from the release could have very serious medical consequences.
This is why I am against the political exaggeration being used in the current Greenpeace campaign. The opposition can simply dismiss their warnings as exaggerations from people who don’t know what they are talking about (which is obviously another political exaggeration, but exaggeration begets exaggeration). This reduces the effectiveness of the very real message that should be communicated to everyone that lives near a chlorine storage facility. It diminishes the message actually communicated to the politicians who don’t have the time to read and interpret Pamphlet 74.