Airbag cushions manufactured from polyester yarn having certain levels of intrinsic viscosity (IV) and certain specific carboxyl end group (“CEG”) levels perform unexpectedly well in high temperature and high humidity environments for airbag cushion applications. This specification identifies certain polyester yarns and characteristics of such polyester yams that are necessary to achieve unexpectedly good performance in such high humidity and high temperature applications. A polymeric polyester yarn for airbags, said yarn having a high degree of tenacity even after a long period of exposure to high temperature and high humidity conditions is disclosed. In one aspect of the invention, a polyester yarn having CEG of less than about 60 equivalents/106 gram, and more preferably less than about 30 equivalents/106 grams, is particularly useful. In another embodiment, a polyester yarn having an IV value of at least about 0.6 performs exhibits superior properties.