Cold and flu season is upon us, and people wary of germs are squirting blobs of hand sanitizer on their hands.

Do those alcohol-based gels really kill 99.99 percent of microbes, as some manufacturers claim? And are they the best way to stay healthy?

The American Chemical Society, a nonprofit science organization, breaks down the science in a new video.

Effective sanitizers generally contain one of three types of alcohol: ethanol, isopropanol, or n-propanol. They work by dissolving the "outer coats" of bacteria and viruses, destroying them in the process, the video explains.

But do these germ-killers contribute to the problem of resistant "superbugs"? That is indeed a problem with the overuse of antibiotics, as bacteria tend to evolve new defenses against those drugs over time, rendering them less effective.

Yet this is not thought to be a problem with alcohol-based hand sanitizers, which are sort of like the nuclear option in germ warfare. There is no evidence that bacteria and viruses can mount any kind of defense against having their outer coats dissolved, according to the American Chemical Society, which co-produced the video with PBS Digital Studios.

Be sure to use a sanitizer that is alcohol-based, however. Some contain antibiotics, which would contribute to the resistance problem.

So how about that those 99.99 percent claims?

The American Chemical Society, which is chartered by Congress and publishes peer-reviewed journals, says those results are based on laboratory tests.

"Real life is messier," the video's narrator states. "The effectiveness of hand sanitizer varies on how oily or dirty your hands are, how much alcohol is in there, and which germs you're actually talking about."

The bottom line: real-world results are often less than 99.99 percent.

The U.S. Centers for Disease Control and Prevention adds that sanitizers should contain at least 60 percent alcohol, and even then, they may be less effective when hands are "visibly dirty or greasy."

What to do? Those gels are handy in a pinch. But there is always soap and water.

Even Purell, a leading maker of hand sanitizers, includes this quote on its website from the U.S. Food and Drug Administration:

"Washing with plain soap and running water remains one of the most important steps consumers can take to avoid getting sick and to prevent spreading germs to others."