The difference between the lower and upper flammable limits, expressed in terms of percentage of vapor or gas in air by volume, and is also often referred to as the "explosive range"
US Department of Labor, Occupational Safety & Health Administration (OSHA), OSHA Office of Training and Education, Glossary of Terms, Construction Safety and Health Outreach Program, DC, USA 1996. http://www.osha.gov/doc/outreachtraining/htmlfiles /hazglos.html, Washington, (OSH - Amer) {516}