Health and Safety

In late June an explosion rocked Schweitzer Hall, which houses chemistry and bio-chemistry labs on the University of Missouri campus, injuring four students and lab techs.¹ It could have been much worse. It is suspected that combustion of stray gases being used in an experiment, hydrogen being the prominent one of interest, led to the blast. The cause is still being investigated at the time of this writing, but we thought it would be a good time for reviewing safe use and storage of flammable materials in the lab.

Flammable substances are those that can easily catch fire and burn in air. They may be solid, liquid, or gaseous, but this article will focus on liquids because according to Prudent Practices the most common fire hazard in the typical research lab is a flammable liquid or the vapor produced by one.² And for a majority of laboratories, flammable liquids are the most commonly stored material and make up the largest volume of hazardous material.

A quick review of basic fire safety is in order. We are all familiar with the “fire triangle,” the three conditions that must exist simultaneously for a fire to occur: an oxidizing atmosphere, usually air; a source of ignition; and a concentration of flammable gas or vapor within its flammability limits. If any one of these is absent a fire cannot occur. Since air is nearly always present, controlling flammable vapors and gases and eliminating potential ignition sources are the best ways to reduce fire hazards. As flammables are used in most laboratories, strict control of all ignition sources is the primary approach to prevent fires.

Important Physical Concepts of Flammable and Combustible Substances
The National Fire Protection Association’s (NFPA) Code for Flammable and Combustible Liquids, NFPA 30, is an excellent resource and introduction to the hazards of these materials.³ Without getting too bogged down in technical details, there are a few concepts that need mentioning and understanding at the outset—the first is flash point temperature. Flash point is the lowest temperature where sufficient vapor is given off to form an ignitable mixture in air. The flammable range is between the upper and lower flammable limits. Flammable limits are expressed in percent volume in air. Concentrations above the upper flammable limit (UFL) are too rich to burn and concentrations below the lower flammable limit (LFL) are too lean to burn. The most dangerous materials are those with the lowest flash point and widest flammable ranges.

The next important concept is the difference between flammable and combustible materials. Flammable liquids are more dangerous. These are liquids that have flash points below 100°F (37.8°C). Combustible liquids have flash points between 100°F and 200°F (93°C). NFPA 30 further classifies flammables as Class I and divides them into Class IA, IB, and IC. Class IA and IB both have flash points below 73°F (22.8°C) with IA liquids having boiling points below 100°F and IB liquids with boiling points above 100°F. Materials with a flash point between 73°F and 100°F are classed IC. Combustible materials are classified as Class II, Class IIIA and Class IIIB based on flash points as well. Class II materials have flash points between 100°F and 140°F (60 °C), with Class IIIA between 140°F and 200°F and Class IIIB above 200°F. The maximum quantity of flammable and combustible materials that can be stored in the lab is set in NFPA 45, Fire Protection for Laboratories Using Chemicals, and determines the hazard classification of the laboratory as we shall see shortly.⁴