The 57-year-old's death in hospital was announced in late August - just as his trade union was ratifying a deal with UPS on improved heat protections.
In a statement to local media, UPS (UPS.N) said it was cooperating with the authorities as they investigated the cause of death.
"We train our people to recognize the symptoms of heat stress, and we respond immediately to any request for help," it said.
Many European and other usually temperate countries still have no laws establishing maximum work temperatures.
"Climate change is such a paradigm shift that all of us need to rethink these legacy economic assumptions," said NELP's Christman.
Persons:
patchily, Chris Begley, Halshka Graczyk, Graczyk, Chaya, Anastasia Christman, Owen Tudor, Tudor, Jerome Volle, NELP's, David Stanway, Catherine Evans
Organizations:
UPS, Teamsters, International Labour Organization, Chiang Mai University, . Occupational Health, Safety Authority, Employment Law, International Trade Union Confederation, Reuters, Thomson
Locations:
Texas, Paris, Thailand, Myanmar, China, Singapore