by Wal Stern

This is an article printed in 'Fire and Arson Investigator' in December, 1995. Included as well is a summary of the responses received to the article as published in the June, 1996 issue of 'Fire and Arson Investigator'.


In Sydney we have had a number of incidents (fires or explosions) involving aerosol packs, used to control insects ('cockroach bombs').

These pressure packs have been around for some time. In the case of an insect infestation the recommendation is to close up the dwelling as much as possible, and spray the pressure packs where necessary.

In the past the propellant in these aerosol packs have been chlorofluorocarbons (CFCs). They are good propellants, and present no fire danger (in fact, some CFCs are used in fire extinguishers). However, CFCs are now being phased out, as they are believed to be a contributory factor to the depletion of the ozone layer in the upper atmosphere.

Ozone is a nuisance in everyday life (it is one of the causes of rubber gloves perishing, the reason why Aids workers wear double gloves or special gloves), but its presence in the upper atmosphere protects us from harmful ultraviolet radiation, and cuts down on skin cancer. It is believed that CFCs float up into the stratosphere, break down under the influence of ultraviolet light to give chlorine radicals (amongst other things), which react with ozone and destroy it. Hence, we are phasing out our use of CFCs.

So far so good. But what do we replace them with? At least one manufacturer of "cockroach bombs" has replaced those propellants with a mixture of hydrocarbons, predominantly propane, isobutane and butane. The manufacturer proudly notes on the can that they have replaced the CFCs, for environmental reasons, but they have at the same time produced a fresh hazard. When the current pressure packs are emptied, gaseous hydrocarbons are released and sprayed around. In one house I went to, the owner had sprayed 15 cans at the one time, throughout the length and breadth of the house. This left a large volume of potentially explosive gas floating around the house. Propane, by way of example, has flammability limits of around 2 to 10 percent; that is, any mixture between those limits is potentially explosive, only needing a spark to go off.

Of course in houses, there are lots of sources of flames and sparks. The packs warn, in small print, that refrigerators should be turned off, as well as all flames and pilot lights (e.g. gas heaters). I don't think that's good enough. People don't read the fine print. Furthermore, I believe the electricity would have to be completely turned off at the main to stop sparks; these could occur at any time, at a power outlet, a poor connection or a frayed cord.

The substitution of hydrocarbons for CFCs may help the ozone layer, but it is presenting us with a new danger from the point of view of fires and explosions. In terms of fire, the continued and expanding use of hydrocarbon mixtures as propellants represents an enormous potential for danger. It would be better if hydrocarbon propellants were not used. One solution would be to replace such aerosol packs altogether by other spray methods.

A summary of responses received.

The respondents confirmed the dangers noted, and gave instances of fires and explosions in a variety of different situations, as a result of ignition of the hydrocarbon mixtures.

The potential for disaster from this source was highlighted by a note from Shalom Tsaroom- Head, Arson Investigation Unit Israel Police, indicating that aerosol containers holding a mixture of hydrocarbons had been a popular device used by terrorists for the past several years.

A number of respondents gave examples of explosions which occurred when large numbers of insecticides and propellants were released in close proximity to pilot lights, specifically from heaters. It was noted that warnings on labels were not read and/or were misunderstood. In the case of the release of multiple cans, each containing around 35% propane, isobutane and butane, near any naked flame or spark, an extremely dangerous situation obviously exists.

There were also a number of cases cited, where the release of three containers had led to a destructive fire. I was concerned at the number and variety of fires reported to me as the result of just one aerosol container, often not an insecticide. For example, there were reports of single containers, one left on top of a kerosene heater, another on a shelf above the gas stove, overheating, venting, and causing an explosion of the propellant gas. One response reported that a single rusted can had leaked, in the vicinity of a naked pilot light, and caused a fire.

The conclusion appears to be that hydrocarbon propellants, even from a single can may cause explosions and fires, if the propellant gases escape or are let out near flames and sparks. Non-ignitable propellant gases are definitely safer, with respect to fires and explosions, and efforts should be made to use environmentally friendly non- ignitable propellants. In the meantime, aerosol containers with hydrocarbon propellants must be stored away from sparks , flames, heat and water, and released away from sparks and flames. Warnings on containers need to be large, and simple to understand. In the short term, more fires and explosions of this type are to be expected.