Under US law, kids' pajamas from 9 months to size 14 must be flame resistant or tight fitting, both subject to specific standards. Manufacturers treat fabrics with flame retardants or use synthetic fabric that is flame resistant. Flame resistant chemicals include halogenated hydrocarbons, inorganic flame retardants, and phosphate based compounds. These chemicals are dangerous and linked to nervous system damage, reproductive system damage, hormone disruption, and deficits in motor skills, attention, and IQ in children. 100% cotton snug-fitting pajamas are best for safety and comfort. The big yellow tags clearly state whether pajamas are snug fitting or chemically treated - so be sure to read those carefully.