It isn't just labs that suffer in the heat. A little common sense goes a long ways. Don't work them in the heat especially if the humidity is up in the 60's or over. Mine have shade and never get worked if it is very hot. I may take them to a river or lake and just let them go in and swim if they want to. Sometimes they just want to lay in it for a while. As for the one lab that could not take a half mile walk in 90 degree heat I would guess there was more to the story than just a normal walk. I would have no qualms about taking mine for a slow walk in any heat. Many times when it gets to 100 we walk to the creek which is about a half mile and let them jump in to cool off.