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I learned an interesting thing from our geriatrician: there is a powerful correlation between life-expectancy and air conditioning, both across and within nations. This is not simply an artifact of greater wealth (you can afford A/C, so you can afford better medical care). Air conditioning matters! The reasons are not entirely known, but there are other indicators that heat is not good for us.

In a different field, studies indicated that the brain functions optimally for learning at a fairly cool temperature. I am no longer certain of the exact temperature, but I believe it was 68 degrees. (Put your sweater on and learn something.) Notice that it is air temperature, not necessarily body temperature, which matters. Another example of air temperature, rather than body temperature, being significant is found in the care of respiratory patients, like me. We breathe much better at fairly cool temperatures. I do best at 66 degrees, and do well up to 69 degrees. I start getting sick above 75 degrees. I cannot tolerate indoor air above about 82 degrees. My outdoor tolerances depend on other factors, especially humidity. I can't help but think there are similar respiratory benefits correlated with temperature for the healthy, especially those living in areas with poor air quality.

We all know that people die during heat waves, with infants, the chronically ill, and the elderly being at greatest risk. Perhaps part of the increased life-expectancy associated with air conditioning is simply that people survive more annual heat waves, giving them more years to die of something else. The air conditioning correlation, however, is independent of latitude. It is good to have A/C, even if your ambient climate does not include a lot of intense heat days each year, or if the hottest days are cooler than elsewhere.

Besides, life-expectancy is often determined by life-long health practices. Heat injury is not coded as a very common cause of death, even among the elderly. A/C appears to offer some other, less obvious, benefits than keeping us alive during heat waves. Of course, it is possible that there is a lack of insight in coding individual deaths. Individuals may die of heat related injuries, which are coded as cardio-vascular, when others around them are not dropping like flies.

Heat damage can be cumulative over the course of many days. We need time to cool-off, not merely time to rehydrate. Becoming a little overheated while out in the yard can start a deadly downward spiral if there is not a good cooling-off afterward. If you cannot really dissipate the excess heat, even during the night-time hours, you begin the following day at a disadvantage.

Aside from the direct problem of heat, there is the issue of hydration. Although closely linked, overheating and dehydration are separate problems, and either can get you in the absence of the other. We all know that by the time you feel thirsty, you are already behind the curve. Unlike hunger, which should tell you when intake is needed for maintenance, thirst is akin to pain (though it doesn't feel like it). It tells you that something has already gone wrong.

Most people don't appreciate how long it can take to recover from dehydration. If you become short a half-gallon, you cannot chug a gallon, grin, and say, "problem solved." Before long, you will have passed most of the extra, 'catch-up' water. You might have made up for your deficit by only a half-cup. It can take days and days to restore your body, as you drink extra, and pass most of it.

So, with fairly high stakes, and thirst as an unreliable early warning system, how do you make sure you drink enough? There are all kinds of fixed-quantity recommendations and formulae out there, but they are one-size-fits-all. To ensure public safety, they tend to recommend the highest amount likely to be needed by an adult. These don't consider weight, physical condition, kidney function, activity level, etc.

Fortunately, you don't really need lab tests every few hours to monitor your hydration. Your urine should appear pale to clear in a normal toilet. If it is a light to medium yellow, nip it in the bud with some extra fluids. If you see medium to dark, take in fluids, and ease off fluid-loss activities until it looks a little better. If it is dark yellow to orange, you are getting in trouble. This should command as much of your attention as treating a cold. If you have special conditions, you might ask your doctor or pediatrician if there is a different standard you should use.

When you really need to rehydrate, you might need to replace some salts, too. This is dangerous territory, especially for infants, the elderly, and other vulnerable groups. You need the right balance of sodium and potassium, and other salts, in your body for your nervous system, including the brain, to work properly. You could make things better or worse by just using a salt shaker, or by just eating bananas.

Balanced oral rehydration formulas are best. Gatorade is close, but includes additional nutrients for a laboring athlete. There are not, to my knowledge, better researched or formulated sports drinks than the original Gatorade. Pedialyte, for all ages, is preferred by our geriatrician (also board certified in internal medicine). The problem is that no adult who still has taste buds can choke it down.

I made a discovery, though. Pedialyte's Popsicles are quite tolerable. The grape is even kind of good, though I don't usually like grape. Neither drinking unfrozen Popsicles, nor freezing regular Pedialyte, has worked as well for me.

Unrelated to rehydration, which should never include the diuretic alcohol, I will warn you that Vodka in Pedialyte won't make it palatable, either. I can't think of another liquor that might mix well with the stuff. I have no bartending skills, however (as evidenced by the fact that I tried Vodka in Pedialyte). I only know this because I had an excessive amount of it on hand and tried to find some way to drink it. I'll not embarass myself with tales of my other attempts.

So, turn on the A/C, whatever your age. Check on folks who don't get out much, or who live alone. If possible, swing by and visit in person so you can feel their air-temperature yourself. There are a lot of people who cannot afford adequate cooling in the summer, after shelling-out for heating in the winter. If this means you, your priorities are, contrary to ambient practice and wisdom, misplaced. You can always put on enough to keep warm, but you cannot always take off enough to keep cool. Use the A/C in the summer, and bundle-up in the winter.

Fortunately, our utility companies are beginning to be merciful in the summer, not just in the winter. Low income assistance is now available in many places, even in the northern states, for cooling bills as well as heating bills. The thinking has long been that everyone needs heat, but cool is a luxury. Even our ancestors built fires, but not until this century did we have air conditioning. That's true, but our ancestors did not enjoy a modern life-expectancy, either.