Temperature is absolutely a problem. Without getting too deeply technical, a heat index above 99F/37C is dangerous even for healthy adults. Las Vegas this week has seen temperatures up to 120F. The forecast today is for a temperature high of 118F/48C (low of 90F/32C overnight), with a relative humidity of 8%. That works out to be a heat index of 111F/44C.

Where I am, today's high will be 82F, but humidity is sitting at 90%, which is a heat index of 92F.

You can also look at wet bulb temperatures; at a certain point, your body can't cool fast enough through evaporative cooling, and you'll die from heat.