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MAPP gas is liquefied petroleum gas mixed with methylacetylene-propadiene. MAPP is a tradename.
The gas is used for welding due to its high combustion temperature of 2927 °C (5301 °F) in oxygen. Although acetylene has a higher welding temperature (3160 °C), MAPP is has the advantage that it requires neither dilution nor special container fillers during transport, allowing a greater volume of welding gas to be transported at the same given weight. Also, MAPP is advantageously used in underwater welding, which requires high gas pressures (under such pressures acetylene decomposes to explosive hydrogen and carbon, making it dangerous to use.)
MAPP is colorless in both the liquid and the the gas forms. The gas has a pronounced garlic odor at concentrations above 100 ppm and is toxic if inhaled at high concentrations.
See [1] (http://www.cdvkiln.com/popmappa.htm).
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