 Hydrogen gas forms explosive mixtures with air in concentrations from 4 – 74 % and with chlorine at 5 – 95 % . The explosive reactions may be triggered by spark , heat , or sunlight . The hydrogen autoignition temperature , the temperature of spontaneous ignition in air , is 500 ° C ( 932 ° F ) . Pure hydrogen @-@ oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye , as illustrated by the faint plume of the Space Shuttle Main Engine , compared to the highly visible plume of a Space Shuttle Solid Rocket Booster , which uses an ammonium perchlorate composite . The detection of a burning hydrogen leak may require a flame detector ; such leaks can be very dangerous . Hydrogen flames in other conditions are blue , resembling blue natural gas flames . 
