Saturday, October 14, 2017

Common Chemical Accident Causes Building Evacuation

Yesterday a 49-story office building in downtown Chicago was evacuated when a common chemical accident occurred on the roof of the building resulting in the release of chlorine gas. Six people were injured severely enough to be transported to local hospitals.

The Incident

Very little information is available in the news reports on the incident (here, here, here and here). The common thread is that “chlorine and acid were accidentally mixed on the roof of the building”. Based upon that this is likely what happened.

A maintenance crew was cleaning/disinfecting the water side of the cooling tower for the building HVAC system. These systems have been implicated in a number of Legionnaire outbreaks, so the cleaning/disinfection of these roof top systems is a fairly normal maintenance task. The ‘acid’ was likely muriatic acid (dilute hydrochloric acid); it is commonly used for pH adjustment, and cleaning metal or concrete. The ‘chlorine’ was almost certainly a solution of sodium hypochlorite (bleach); it is commonly used as a disinfectant and cleaning solution.

In disinfecting a small body of water the muriatic acid is added to lower the pH of the water. Then the chlorine is added to kill off bacteria. With adequate mixing or an appreciable time between adding the two chemicals to the water there is no problem. If the two chemicals are added in close physical or temporal proximity the they remain concentrated enough to allow a very quick exothermic action to occur. A byproduct of that reaction is the release of chlorine gas.

Unless someone is really stupid in the amount of bleach added to the water, there will not be enough chlorine gas released to kill anyone unless they are in a small, confined space above the surface of the water. Relatively small, non-fatal, amounts of chlorine gas will cause severe irritation to the eyes, nose and respiratory tract. Prompt medical evaluation is routinely recommended for anyone experiencing eye pain or difficulty breathing after relatively minor chlorine gas exposures.


In hind sight, there was almost certainly no need to evacuate the building. The amount of chlorine gas released would not have been medically significant beyond the immediate are of the release on the roof. I suspect that enough gas got into one of the HVAC air intakes to allow some people to detect the odor of chlorine (detectable by the average person at very low levels). Complaints of a strange chemical odor reported in the building coupled with the report of a chemical incident on the roof would be sufficient, however, to make any emergency response incident commander order a precautionary evacuation.

One of the reasons for this is that the same reaction between hypochlorite and muriatic acid make for a pretty interesting improvised chemical munition in closed quarters like a building. Without the diluting effect of a small body of water, the fast and strong exotherm results in low order explosion (no flame but and expanding gas cloud) that releases chlorine gas. Both chemicals are easy to buy and the only difficulty in constructing these bombs is how to keep the two chemicals apart until you want the reaction to take place. Again, unless the ‘bomb’ is really large, there is little real danger outside of the immediate area of ‘detonation’, but the loud bang and chlorine odor will do a nice job of starting a panic in a crowded building.

I expect that the investigation of this incident will ultimately place the blame for this incident on ‘human error’ and inadequate training of the maintenance personnel. People really can handle these two relatively innocuous industrial chemicals safely with just a modicum of training and supervision. But, the reason that this is such a common accident is that the process looks so simple and the chemicals look very common, so no one really takes the safety issues seriously until it is too late.

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