The blankets keep you warm thanks to their design. As an impermeable metallized plastic film, they capture up to 90% of the radiated body heat that would normally be released into the environment. So they warm us mainly with the heat that we constantly generate and lose anyway!

An emergency blanket (also known as a Mylar blanket, rescue blanket, first aid blanket, safety blanket, thermal blanket, weather blanket, heat blanket or shock blanket) is a particularly light and low-volume blanket made of heat-reflecting material, thin
Aluminum foil. Their design reduces heat loss in a person's body that would otherwise occur through thermal radiation, water evaporation or convection. Their light weight and compact size before unfolding make them ideal when space or weight is at a premium. They can be included in first aid kits and camping equipment. Lost campers and hikers have another potential advantage: the shiny surface flashes in the sun, allowing them to be used as an improvised distress signal
for searchers and as a signaling device for other people over long distances.