the hands are greasy or visibly soiled. In hospitals, the hands of healthcare workers are often contaminated with pathogens, but rarely soiled or greasy. In community settings, on the other hand, grease and soiling is common from activities such as handling food, playing sports, gardening, and being active outdoors. Similarly, contaminants like heavy metals and pesticides (generally found outdoors) cannot be removed by hand sanitizers. Hand sanitizers might also be swallowed by children, especially if brightly colored. Some commercially available hand sanitizers (and online recipes for homemade rubs) have alcohol concentrations that are too low. This makes them less effective at killing germs. Poorer people in developed countries and people in developing countries may find it harder to get a hand sanitizer with an effective alcohol concentration. Fraudulent labelling of alcohol concentrations has been a problem in Guyana. Schools The current evidence for the effectiveness of school hand hygiene interventions is of poor quality. In a 2020 Cochrane review comparing rinse-free hand washing to conventional soap and water techniques and the subsequent impact on school absenteeism found a small but beneficial effect on rinse-free hand washing on illness related absenteeism. Health care Hand sanitizers were first introduced in 1966 in medical settings such as hospitals and healthcare facilities. The product was popularized in the early 1990s. Alcohol-based hand sanitizer is more convenient compared to hand washing with soap and water in most situations in the healthcare setting. Among healthcare workers, it is generally more effective for hand antisepsis, and better tolerated than soap and water. Hand washing should still be carried out if contamination can be seen or following the use of the toilet. Hand sanitizer that contains at least 60% alcohol or contains a "persistent antiseptic" should be used. Alcohol rubs kill many different kinds of bacteria, including antibiotic resistant bacteria and TB bacteria. They also kill many kinds of viruses, including the flu virus, the common cold virus, coronaviruses, and HIV. 90% alcohol rubs are more effective against viruses than most other forms of hand washing. Isopropyl alcohol will kill 99.99% or more of all non-spore forming bacteria in less than 30 seconds, both in the laboratory and on human skin. In too low quantities (0.3 ml) or concentrations (below 60%), the alcohol in hand sanitizers may not have the 10–15 seconds exposure time required to denature proteins and lyse cells. In environments with high lipids or protein