For cleanest hands, squirt and count to 30

BOSTON — You’ve heard the advice: For clean hands, scrub with soap and water for as long as it takes to sing “Happy Birthday” twice. But is hand sanitizer just squirt and go? Finally, there’s a scientific answer: To kill bacteria, rub for at least 15 to 30 seconds. After 45 seconds, you’re not doing much more good.

Proper hand hygiene is the most important means of infection control in hospitals and clinics. But health care workers follow hand washing guidelines only about 40 percent of the time, studies show. Compliance is higher with hand gels — which kill microbes with a combination of alcohol and friction — perhaps because a squirt of gel is easier to apply.

To provide more evidence-based guidance on hand sanitizer use, scientists from Geneva University Hospitals in Switzerland used E. coli bacteria to contaminate the hands of 23 health care workers. Then each person received a 3-milliliter squeeze of hand sanitizer. Participants were instructed to rub for different amounts of time, ranging from 10 to 60 seconds. The concentration of bacteria plunged after 10 and 15 seconds of friction, and then dropped slightly more after 30 seconds. But significant reductions in bacteria stopped at the 45-second mark — a curious finding that researcher Daniela Pires says she and her colleagues cannot explain.

The research was presented June 18 at ASM Microbe 2016, a meeting of the American Society for Microbiology and the Interscience Conference on Antimicrobial Agents and Chemotherapy.