Modern flame retardants should not be toxic to humans, animals or plants.
They should not migrate, i.e. should not be released from the finished product through evaporation. This is achieved best by using reactive flame retardant that react within the polymer.
In addition, modern flame retardants should not release additional toxic or corrosive smoke gases during a fire.
The recycling properties of the product should not be negatively affected.
Flame retardants must be enviromentally-friendly, i.e. either neutral or bio-degradable.