Acetylene is not especially toxic, but when generated from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinct garlic-like smell. It is also highly flammable, as are most light hydrocarbons, hence its use in welding. Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen.
Acetylene is not highly toxic but when produced from calcium carbide it may contain harmful impurities like phosphine and arsine that give it a distinct garlic-like odor. It's also flammable, as are most light hydrocarbons, making it suitable for welding. Its primary danger lies in its instability, especially under pressure, where it can react exothermically to form various products such as benzene and/or vinylacetylene, potentially including carbon and hydrogen.