Artificial grass can get hot in the sun, as do other surfaces such as concrete. Ensuring the turf has the correct sand infill (which dissipates the heat into the ground) is the most important factor here to ensure it stays cool. By giving the lawn a quick spray with a hose on very hot days, you will instantly cool the turf. The sand retains the moisture for a few hours, which then evaporates to help dissipate heat.