Would a small hand held heat source upto 45c be enough to stop a homeless person dying in the cold? or stop loss of toes, fingers? we wish to provide usb heaters to the homeless and want to check the medical recommendations about this?

Hypothermia injury: A small single point heater is unlikely to provide protection from cold injury. Unfortunately extremities would require insulation and core temperature maintenance is best managed with warm fluids and core insulation. A heavy parka and mittens would be more effective. The data comes from winter mountaineering where a small heat source is not effective without shelter and ground insulation.