For most people, summer involves numerous daily shifts between scorching outdoor heat and frosty air-conditioned interiors. But does exposing the body to extreme temperature swings make people sick? Professor Ron Eccles, director of the Common Cold Centre at Cardiff University in Wales, which performs clinical trials for treatments for coughs, colds and flu, explains why keeping a sweater at work isn't such a bad idea.

As warm-blooded animals, humans are hard-wired to keep our body temperature at around 98 degrees Fahrenheit. So when a person is exposed to frigid environments after being in the summer heat, the body 'will do whatever it can to defend itself against chilling,' Prof. Eccles says.

One such defense: A thermal regulator in the brain, after receiving messages from temperature sensors in the skin, automatically alerts blood vessels there to constrict. 'You can see this when someone suddenly goes into a very cold building, they go pale or their skin mottles,' says Prof. Eccles. The next stage is shivering, which will raise body temperature by generating heat.

At the same time, blood vessels constrict in the nose and throat, where bacteria and viruses often lurk. 'If you were to look into a throat, you could see it go from a nice pink-red to a very pale color,' says Prof. Eccles. 'This happens within a few seconds to conserve the heat that we lose to the air we breathe out.'

When blood flow diminishes, the white blood cells that typically fight bacteria and viruses do too, allowing these latent risk factors to easily bloom into a full-blown cold. 'If there isn't as much blood flow to the throat, there aren't enough white blood cells to ward off infection,' Prof. Eccles says.

But, he emphasizes, a cold won't develop unless the bacteria or virus is already present in the body.

Sweating can amplify the risk by keeping the body colder longer-and making it harder to regain an optimal core temperature. Having sweaty clothes when walking into an air-conditioned building 'could feel like diving into an ice-cold swimming pool,' says Professor Eccles.

Getting some of that perspiration to evaporate first, by sitting in the shade for a few moments, for example, can help. So can keeping a sweater or light jacket at the office to ease the temperature transition.

Heading in the other direction-from the deep freeze to the steamy outside-doesn't pose the same cold risk, says Prof. Eccles, since even more blood circulates to the skin, nose and throat. 'It's a lot like when your car radiator goes on when it gets too hot, to cool off,' he says. 'If you're very overheated, much of your five liters of blood gets distributed to the skin-turning you red-to eliminate heat.'

It can take the body only a few seconds or minutes to acclimate to the cold inside, says Prof. Eccles.