Maybe you keep a bottle in your car or in your purse. Or you grab a splurt as you check out of the grocery store or leave the doctor's office. Hand sanitizer is one way we try to arm ourselves against the onslaught of germs we face in the world.

But how effective is hand sanitizer? It depends on the alcohol content and the germs you're hoping to tackle. In most case, you'd probably be better off washing your hands with soap and water if you can, say the hygiene experts.

How we get sick

man opening microwave oven It's those hands and fingers that are the problem; they're always touching things, especially shared surfaces like knobs. (Photo: Andrey_Popov/Shutterstock)

About 80% of infectious diseases are transmitted by direct and indirect contact, microbiologist and germ expert Dr. Philip Tierno tells MNN. We pick up germs directly when people cough, sneeze or talk near us. We get them indirectly when a sick person touches a doorknob, remote control or elevator button, then we touch it and touch our mouth, eyes, nose or an open cut.

"The ten dirtiest things on the human body are the ten fingers," say Tierno, professor of microbiology and pathology at NYU School of Medicine and NYU Langone Medical Center. "No matter how dirty your hands are, you can protect yourself with regards to indirect transmission and your hands."

And the best way to do that is by washing your hands.

The argument for hand-washing

man washing hands Don't just run your hands quickly under the water. (Photo: fizkes/Shutterstock)

When you wash your hands, it isn't the soap that's killing the germs, but it's the rubbing back and forth and the action of washing that is removing the germs, says Tierno. The fewer organisms that you have left, the lower your risk of getting sick.

"It's all about numbers," he says. "For salmonella, you need to take in a minimum of 10,000 bacterial cells to disrupt your normal floral in your bowel. For shigella, that may require only 10 to 100 cells to overcome your defense. For a norovirus, the so-called stomach flu, that may require just one virus particle to create a problem."

That's why the Centers for Disease Control and Prevention (CDC) recommends washing with soap and water whenever possible as the first line of defense against all kinds of germs. Hand-washing gets rid of most types of germs, including Cryptosporidium, norovirus and Clostridium difficile, which typically aren't removed by hand sanitizers.

In late January 2020, the Food and Drug Administration (FDA) warned the makers of Purell hand sanitizer to stop claiming that the product could stop people from catching illnesses, including the flu, Ebola and norovirus. The agency said it was not aware of "any adequate and well-controlled studies" that support claims that killing or decreasing the number of bacteria or viruses on the skin by a certain degree produces a corresponding reduction in infection or disease caused by such bacteria or viruses.

Among a list of specific issues in the warning letter, the FDA pointed out claims on the company's website and social media. The claims included Purell "kills more than 99.99 percent of the most common germs that may cause illness in a healthcare setting, including MRSA" and "Purell Products are proven to reduce absenteeism."

In addition, hand sanitizers aren't as effective when hands are visibly dirty or greasy, says the CDC.

The key is to make sure you wash your hands thoroughly. The CDC says that means lathering them up and rubbing them together for at least the time it takes you to sing the "Happy Birthday" song. The World Health Organization (WHO) says you should be much more detailed than that. Check out both methods here.

When and how to use hand sanitizer

hand sanitizer at hospital Hand sanitizer is convenient in hospitals where providers see so many patients. (Photo: ALPA PROD/Shutterstock)

Sometimes you just don't have access to soap and a sink. In those instances, hand sanitizer is a smart choice.

Effective hand sanitizers contain alcohol. It's the alcohol in the product that kills many common organisms, says Tierno. (They also should never contain triclosan, which was banned in liquid hand soap but still shows up in some consumer products.)

The CDC also points out that many studies show that sanitizers work well in clinical settings like hospitals, where hands come into contact with germs but generally are not very dirty. And Consumer Reports says that hand sanitizers can be a quick and easy choice for workers who have to clean their hands before and after contact with every patient when hands aren't visibly dirty.

But not all hand sanitizers are created equally. Choose one that is at least 60% alcohol, recommends the CDC. That's more effective at killing germs than a sanitizer with less or no alcohol concentration.

To use hand sanitizer correctly, Tierno says you should use at least a quarter-sized dollop of gel.

Apply it to the palm of one hand and rub your hands together. Rub the gel all over your hands, in between your fingers and into your nail beds. Rub until your hands are completely dry. Don't wipe it off before it dries. The process should take about 20 seconds.

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

How helpful is hand sanitizer?
Hand sanitizers aren't as effective as soap and water, but sometimes they're a smart choice.