Was it hand hygiene, fragility of the patients, or room cleaning procedures?

Tuesday, October 29th, 2019

Harvard Medical School graduate and lecturer Dr. Stephanie Taylor and colleagues studied 370 patients in one unit of a hospital to try to isolate the factors associated with patient infections:

They tested and retested 8 million data points controlling for every variable they could think of to explain the likelihood of infection. Was it hand hygiene, fragility of the patients, or room cleaning procedures? Taylor thought it might have something to do with the number of visitors to the patient’s room.

While all those factors had modest influence, one factor stood out above them all, and it shocked the research team. The one factor most associated with infection was (drum roll): dry air. At low relative humidity, indoor air was strongly associated with higher infection rates. “When we dry the air out, droplets and skin flakes carrying viruses and bacteria are launched into the air, traveling far and over long periods of time. The microbes that survive this launching tend to be the ones that cause healthcare-associated infections,” said Taylor. “Even worse, in addition to this increased exposure to infectious particles, the dry air also harms our natural immune barriers which protect us from infections.”

Since that study was published, there is now more research in peer-reviewed literature observing a link between dry air and viral infections, such as the flu, colds and measles, as well as many bacterial infections, and the National Institutes of Health (NIH) is funding more research. Taylor finds one of the most interesting studies from a team at the Mayo Clinic, which humidified half of the classrooms in a preschool and left the other half alone over three months during the winter. Influenza-related absenteeism in the humidified classrooms was two-thirds lower than in the standard classrooms—a dramatic difference. Taylor says this study is important because its design included a control group: the half of classrooms without humidity-related intervention.

Scientists attribute the influence of dry air to a new understanding about the behavior of airborne particles, or “infectious aerosol transmissions.” They used to assume the microbes in desiccated droplets were dead, but advances in the past several years changed that thinking. “With new genetic analysis tools, we are finding out that most of the microbes are not dead at all. They are simply dormant while waiting for a source of rehydration,” Taylor explained. “Humans are an ideal source of hydration, since we are basically 60% water. When a tiny infectious particle lands on or in a patient, the pathogen rehydrates and begins the infectious cycle all over again.”

This isn’t exactly news though.


  1. Eli says:

    Seriously? You mean, like, air conditioning? Whudathunk! It isn’t as though sinus illnesses increase in air conditioning season, right?

  2. Graham says:

    Funny, I’ve hated humidity all my life and still do. It’s gross, and sickening in its own ways, in terms of its enervating effects. Whether and how it contributes to sickening in the sense of actual disease, I’ll leave to the tropical hygienists. Other than mosquitoes, whose baleful influence is known.

    So when it’s above 26-27 Celsius and above 50-60% humidity, I want my indoor AC if I can get it. Living without it for about 7 Ottawa summers was unpleasant. Especially the magical summer of 2018 when it was like a swamp in this valley for two months. I know, the whining of a temperate zone man.

    But, my love of cool doesn’t come with as much love of dry as it used to. I used to revel that my old apartment was kept above 23 degrees Celsius [sometimes 25] in winter by its crappy hot water radiators, and with humidity under 30%. Only when it had gone down to 20 or less was I concerned.

    So a couple of winters ago I started having chronic low-degree visual blur and constant eye itching. By the end I thought I had some sort of horrid eye disease and it felt one night like some tiny lifeform was gouging its way out of my skull through my eyeballs.

    I’d never had dry eyes before. Had no idea it could get that bad. The optometrist’s first words on finishing her inspection were, “You have few tears”.

    I resisted the temptation to say I’d wept them all. She just meant my eyes had grown a tad dry. A big tad.

    Between that and dried up sinuses, I think that apartment was trying to kill me with dryness in winter, dampness in spring and fall, and humidity in summer.

    But it gave me a new appreciation of the perils of dry air in any environment. I got more colds toward the end, too.

    You’d think health care practitioners would have known this, even if they didn’t have the most up to date models of microbial life cycle as revealed here.

    Not that I’d want to be in an un-air-conditioned hospital in July with a case of swamp crotch on top of whatever put me in there. But there needs to be a happy medium of interior environment.

    I get it’s hard in large buildings with a lot of rooms and variable size spaces, but even office buildings of some age seem to manage the HVAC somewhat tolerably.

    My office this morning in a decaying 40-year-old building with many systemic problems was nonetheless at 21°C and 60% humidity as measured by an analog thermo/hygrometer. It mostly retains those conditions year round regardless of the outside. Which as it happens today is reported to be 15°ree;C and 69% humidity outside. A relatively dry day compared to most of our recent fall weather.

    Hospitals have a lot more doors opening and closing, frantic movement of people and equipment, and so on. But it should still be possible, and judged highly important, to keep the climate controlled.

    Maybe they all just loved dry air as I once did.

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