There are three reasons for ventilation: 1) for toxic airborne chemicals, 2) to prevent a build-up of flammable gases or vapors, and 3) for comfort of the inhabitants of the area. Since health effects of chemicals occur at air concentrations well below the lower explosive limits of solvents and gases, then if one ventilates to prevent health effects, one also prevents a buildup of vapors that could catch fire or explode. CFR 1926.27 Ventilation Flammable and combustible materials can ignite in the presence of an ignition source such as a spark. Flammable solvents, combustible organic dusts, metal dusts, or ignitable fibers are examples of such materials. Flammable solvents are a fire or explosion hazard if the solvent vapor concentration in air is greater than the lower explosive limit (LEL) or less than the upper explosive limit (UEL).