Definition of toxic, flammable and explosive substances

The amount, and the physical and chemical characteristics, of substances utilized in an industrial activity or services (whether proposed or in operation), as well as those being transported, are the most important factors in defining their degree of risk. Therefore, lists have been established for regulating dangerous substances (principally toxic, explosive and flammable), for which reported or controlled amounts have been established.

A substance is considered acutely toxic according to a concentration capable of producing death of half or 50 percent (LC50) of animals exposed via inhalation for eight hours at an amount of 0.5 mg /liter of air; or, a skin exposure dose in the amount of 50 mg / kg of body weight (LD50); and/or, an oral dose equivalent to 25 mg / kg body weight (also LD50). In the absence of these values, the measure used is any lower concentrations or doses that are lethal in any animal testing.

These substances form a mixture with air in concentrations that can ignite a flame spontaneously or by the action of a spark. The concentration of the mixture is considered equivalent to the lowest limit of flammability. These substances are considered flammable if they have a flashpoint below 60° C, absolute vapor pressure not exceeding 2.81 kg /cm² and boiling temperature of 37.8° C. The National Fire Protection Association (NFPA) defines a flammable liquid as one with a flash point below 37.8° C. It establishes the following sub-classifications NFPA for these liquids:

 

These substances produce a sudden, turbulent expansion, caused by the ignition of a flammable vapor of a certain volume, accompanied by noise, with violent physical forces that could seriously harm structures by the rapid expansion of gases.

 

In Mexico, for the calculation of reported or controlled amounts, the reference for emission sources is the surrounding 100 meters. In the case of toxic substances, the limits are the same as IDLH (Immediately Dangerous to Life or Health) concentrations. In the case of flammable substances, the measure is thermal radiation equal to 5 kw/m², and for explosive substances the measure is excess pressure at 0.5 psi.

 

The distance criterion for 100 meters was adopted considering that a gas leak or the evaporation of a toxic or flammable liquid forms an elliptical-shaped cloud, in which the wind can form a concentration of the substance at the end of the ellipse (note: IDLH concentration for toxic substances and / or the concentration equivalent of Lower Explosive Limit, LEL, for flammable substances). For these concentrations, the considerations outlined in the Emergency Action Guide for selection of hazardous materials, published in 1978 by the Department of Transportation (DOT) U.S., are taken into consideration.

 

Each country includes substances in its lists by taking into account other factors such as the existence of substance, its use, how often it has been involved in accidents and the severity of the impact of these accidents.

 

In the U.S., the list of substances regulated under Section 112 (r) of the Clean Air Act includes 77 toxic substances. Sixty-three flammable and explosive substances have been identified by the U.S. Department of Transportation.

 

For its part, the Directive of the Council of the European Union enacted in 1982 brings together 180 toxic, flammable and explosive substances.