For additional information on this subject, please refer to
Car Dangers.
Dogs do not tolerate warmer temperatures as well as humans. They depend upon rapid breathing to exchange warm air for cool air. Accordingly, when air temperature is close to body temperature, cooling by rapid breathing does not work well. Dogs with airway problems have an especially difficult time in the heat. It is important to remember this when traveling by car with your dog. Leaving him in a closed or partially closed automobile on a sunny day, even if the temperature is moderate, is dangerous, since the car can quickly heat up to 100 degrees F(37.7 C) or more, which can result in death, by suffocation, very quickly. So, even though your dog loves to ride with you and the "look" you get when leaving him home seems unbearable, it is wisest to not take him with you. An errand that is supposed to take "only a minute" could easily turn into a half hour or more. If all this talk about heatstroke and cars seems like an exaggeration, park your car on a warm, summer day, even in the shade, and roll up, or keep cracked slightly, the windows, as you would if you left your dog alone. Then, just sit. It shouldn't take long before you begin to experience what the dog goes through while in there.
So, unless you plan to be taking your dog simply for a ride without stopping anywhere and leaving them in car, please use good judgement and leave them home.
Also, use common sense when exercising or just letting your dog out in the heat and humidity. It's best to exercise them in the cool of the morning or after the sun goes down. If taking your dog to an outdoor event, make sure that he has lots of water and a shady spot near you to keep cool.
One thing that we do when we are out in the heat is carry a spray bottle
of water (ice in it if neck is big enough) and we continously spray
head, stomach and the pads. Male dogs get it on their two not with the
icy water tho yuk. I found a portable mister at a dog show and we carry
that with us. The puppies will fight to get to the front of a crate to
get their bellies sprayed. It is really cute to see them stretch longer
to get wet. Then I am the one who believes in super soakers to correct
a barking in the crate/pen problem so they are used to being hit with
water. Doggone Dachshunds you really can't please them