Someone - possibly a woman I used to work with who volunteered at a local Scout group - told me about these cooling neck scarves:

Cool Scarf

"The N-rit Cool Scarf neck cooler rapidly lowers body temperature
Stays cool and does not require refrigeration. How to use: Soak your handy kooler in cool or cold water for about 30 minutes or until the crystals have expanded."

I'm not sure what the crystals are. They are described as "crystal polymer cooling technology", whatever that is.

In Oz we are very aware of not doing heavy physical activity during heat waves. When I first visited the lovely town of Broken Hill in Central Australia on an extremely hot summer day, my friend and I were walking around the shopping area at midday and when we went into the shops the shop attendants were emerging from their back rooms looking a bit sleepy.

We found out that the canny locals tend to have siestas in the middle of the day and it was only the crazy tourists, e.g. us, who were silly enough to think that life goes on as normal at the hottest part of the day. If you can, save your physical activity for a cooler part of the day, or better still when the heatwave is over.

Even the workers in Australia are told to stop working in extreme heatwaves and any day with temperatures over 37 deg Celsius is considered too hot to work according to Occupational Health & Safety standards.