Heat rash occurs when sweat ducts become clogged. (webmd.com)
Antiperspirants use compounds — such as aluminum chloride, aluminum chlorohydrate and aluminum zirconium — to form a temporary sweat duct plug. (scientificamerican.com)
Babies tend to get it more often than older children do because their immature sweat ducts rupture more easily, trapping perspiration under the skin. (parents.com)