The ever so slightly shallow side of me equates humidifiers with uncontrollable frizzy hair. Judge me if you like, but let me assure you that I am not alone. I am just one of the millions of women worldwide who share that view point. As proof, I invite you to check out the number of frizzy hair or anti-frizzy hair products that sell like hotcakes.
Like many women I only need and use these sticky products when the humidity is high. As an example, I would never use them when visiting Arizona but use gallons when I am in East Africa.
So what has my hair got to do with anything and why should you care? It turns out that my hair has a lot to do with best practices in humidifiers (all will become clear, I promise).
Humidifiers are being increasingly marketed for a wide range of health and wellness benefits including prevention of flus and colds, managing allergies, rehydrating dry skin etc etc.
Should every house, school and office have a humidifier? Should we include humidifiers in the design of our buildings? What does the science have to say about this?
What Actually Are Humidifiers?
Great question. Let's start first with 'What Actually Is Humidity?'
Humidity refers to the amount of water in a vaporous state contained in a gas. More simply put, my Irish grandmother would have called it 'damp'.
It can be classified as:
absolute ( weight of water in a given volume of gas mg/L
relative (ratio of the weight of water vapour relative to the gas capacity to keep water at a specific temperature).
The capacity of the air to hold water increases as the temperature increases. Think of how people sweat (and how bad their hair looks) when in the Amazon jungle.
If the water content in the air exceeds the holding capacity of the air, then the water condenses.
Usually, inhaled air is humidified in the upper airways.
More than a century ago airway damage was noted in people who inhaled dry gases during artificial ventilation. This is especially important when gases bypass the hydrating effects of the upper airways.
What Actually Are Humidifiers?
Humidifiers are devices that put moisture back into the air or adds water to a gas.
There are two main ways that humidifiers can work:
- active - this uses an external source of heat and water to add moisture to the air
- passive - this uses the patients own body temperature and hydration to add moisture to the air.
They can range from professional grade (like the ones used in ICU) or simple home gadgets.
We are most interested in home humidifiers but will look at both in the article.
There are over 10,000 humidifiers (which are by definition active humidifiers) and related products for sale on Amazon and these range in price from $4 to $4000.
Is There Any Research?
To put this into context, there are over half a million publications on oxygen which includes over 25,000 clinical trials.
Do They Help Dry Skin?
This is debatable.
Two papers written by the same author from Japan explore this topic to some degree.
In the first paper published in 2013, Ohno studied the effect of a mist of water nano-droplets on skin and found that skin moisture and elasticity increased without an increase in air humidity (1). Just to be clear, this was a highly experimental and artificial scenario (which involved testing the skin using suction with and without targeted mist therapy) and is entirely different to the little room humidifiers being sold online.
In the second study published in 2015, Ohno showed that it is easier for mist to penetrate into the skin in winter (2). The proposed explanation for this finding is that corneocyte skin cells become smaller as skin turnover increases in cold environments. Interesting, could it be that the benefit of humifidifiers varies depending on other local and human factors?
A more recent British review looked at papers that contained the keywords skin, skin disease, humidity, eczema and mist (3). This review found that eczema worsened with low humidity and also with high humidity. Again are we maybe getting a sense that the effects of humidity are not linear (meaning that there is not a direct straight line relationship between humidity and outcomes but more of a u-shaped curve?).
The review concluded that 'ambient relative humidity has an impact on a range of parameters involved in skin health but the literature is inconclusive'.
How can I improve on the bottom line from a comprehensive review paper that says 'Ambient relative humidity has an impact on a range of parameters involved in skin health but the literature is inconclusive'.
Do They Reduce Risk Of Colds or Flus?
The question can be looked at two ways.
- Do humidifiers reduce the risk of catching colds and flus?
- Do humidifiers reduce the risk of actually having a colds or flu eg how long you are sick etc?
Let's look at both.
Do humidifiers reduce the risk of catching colds and flus?
We all know from experience that chest infections are more common in the winter months. Why is that? A 2008 study from Finland examined the association between cold temperatures, low humidity and respiratory tract infections (4). This was a population based study.
They studied a total of 643 respiratory tract infection episodes in 892 military recruits. They studied the meteorological data, temperature and humidity in the days preceding the development of respiratory tract symptoms in the military recruits. They found that a 1 degree drop in temperature was associated with a statistically significant increase (2.1%) in the risk of the common cold and a statistically significant increase (2.1%) in the risk of lower respiratory tract infections.
A decrease in 1gm/m-3 in absolute humidity increased the estimated risk of an upper respiratory tract infection by 10% which was highly statistically significant (p<0.001).
Infection control doctors (like me) worry about the spread of infection form person to person. A Stanford University mathematical model found that the key infection control measure for influenza is an appropriate grade face mask (5). That's why you might see people in hospitals wearing face masks during flu season (or even some people on public transport). But what makes an infection go viral (if you excuse the pun) and become an epidemic?
They also found that the combination of a face mask, humidifiers, and ventilation raises the threshold which dictates whether or not an epidemic breaks out.
The CoPanFlu-France is a French cohort study established in 209 to assess the risk factors for infection by the pandemic influenza virus (H1N1pdm) in the French general population (6). They did a very complicated multivariate analysis and found the presence of an air humidifier in the living room was a possible protective factor.
Do humidifiers reduce the risk of having a cold or flu?
A 2011 Cochrane analysis compared the effects of steam inhalation or humidified oxygen to relieve respiratory distress and to evaluate adverse events in children up to three years old with acute bronchiolitis (7). There was only one study of 156 children aged between seven weeks and 24 months with signs and symptoms of bronchiolitis that met the eligibility criteria for inclusion.
This hardly makes for a meta-analysis. The study showed a significant decrease in respiratory distress symptom score in a nebulised salbutamol group but no significant decrease in the score in a mist in a tent or nebulised saline groups. Essentially, we are non the wiser after the meta-analysis.
Another Cochrane meta-analysis just published last year updated a previous review of the use of humidified air for the common cold (8). The meta-analysis was carried out by researchers from Chandigarh, India. They identified six trials from five publications involving a total of 387 participants.
The number of participants reporting resolution of symptoms was not significantly higher in the heated humidified group. There was significant heterogeneity in the effects of heated, humidified air on different outcomes, therefore quality of the evidence was considered to be low. Some studies reported minor adverse events (including discomfort or irritation of the nose).
Humidifiers may help prevent the spread of winter flus and colds but do not seem to do much good helping treat people who already have flus and colds.
Do They Help Asthma and Allergies?
A clinical trial looked at the effect of humidification on mechanical ventilation in adult asthmatics (9). A total of 72 adults were assigned to 6 groups: 3 of which involved different devices with humidification and 3 of which involved the same devices without humidification. The study found no benefit of humidification with any of the devices on the number of ICU days or number of days ventilated.
A 2013 Cochrane meta-analysis looked at dehumidifiers for asthma (10). Yes, dehumidifiers and not humidifiers. This was one of 123 asthma related Cochrane reviews published in 2013. (I had no idea that there were that many Cochrane reviews published per year).
Dehumidifiers are of interest in asthma as a warm humid environment favours the growth of dust mites and mold. Dust mites and mold are common triggers for asthma. This review found just 2 relevant studies and concluded that the was no convincing evidence that dehumidifiers helps with asthma.
A study published just this year looked at data from 5210 children in the El Paso Children's Health Study to determine indoor environmental health risk factors for allergy and asthma (11).
The study involved questionnaires and pulmonary function tests (where available) on fourth and fifth grade schoolchildren living in a major United States-Mexico border city. Humidifier use was positively associated with both allergy and asthma prevalence or in plain English - humidifiers increase the risk of allergy and asthma.
A 2016 study investigated the effects of ethnicity and environmental exposures on eczema in 5765 Hispanic white and non-Hispanic white children who participated in the Southern California Children's Health Study (12). Indoor environmental factors (e.g., mold, water damage, humidifier use) increased the risk of eczema in non-Hispanic whites independent of a parental history of allergic disease, but, in Hispanic whites, increased risks were observed, primarily in children without a parental history of allergic disease.
Neither humidifiers nor dehumidifiers seem to help asthma or allergy and may increase the risk of allergy, asthma and eczema.
Do They Help Snoring?
The only related data comes from studies on sleep apnea. These people have complete or partial obstruction of their airway when sleeping (and tend to snore). Obstructive sleep apnea is associated with a range of health issues including hypertension and lowered immunity. A study looking at humidify air in sleep apnea found that humidity did not affect quality of life but did reduce nasopharyngeal dryness (13).
Humidifiers do not help with snoring.
Are Humidifiers Safe?
A key risk with reusable humidifiers is infection (14)
A systematic review from China evaluated the effects of humidified versus non humidified low flow oxygen therapy. They evaluated a total of 27 trials involving 8876 patients. Non humidified oxygen offers more benefits in terms of reducing bacterial contamination of humidifier bottles.
Infection associated with humidified oxygen is a pretty big issue. High rates of contamination with reusable oxygen humidifier have been seen in medical surgical and ER wards (15). Common causative bacteria including pseudomonas and staphylococci. On the plus side, disposable oxygen humidifiers were not contaminated.
Concerns about infection lead to an even more serious problem with humidifiers. In order to reduce the risk of infection, disinfectants were used to sterilise the equipment. In 2011 investigators in Korea identified what has been now called the Humidifier Disinfectant Disaster. Essentially over 1000 adults and children were noted to have humidifier disinfectant associated lung injury (16)
This ranged from being asymptomatic to full blown respiratory failure requiring lung transplantation and/or resulting in death.
Where does all that leave us? At the end of the day, there are two things we need to consider while passing a verdict on the utility of humidifiers.
- Does humidity matter?
- If so, do humidifiers actually change the humidity?
1. Does humidity matter?
A review paper found (that I saved til now because it is so good) looked at the effects of relative humidity on respiratory tract infections and allergy (17). This paper found 9 epidemiological studies that involved schools, houses and offices.
They found that absenteeism and respiratory infections were lower in people working in mid-range humidity and the rates were higher in people in low or high humidity environments. So yes, humidity matters but it is all a question of moderation. We neither want super low nor super high humidity. Just mid-range humidity.
Back to hair. Hair also responds to humidity. We see more unruly frizzy hair as the humidity increases. That is why many women use anti frizz agents in warm humid climates but not in dry desert conditions.
This is exactly why we can't make blanket recommendations that humidifiers should be used for this, that or the other. It depends entirely on the local conditions. And it also depends on whether indoor humidifiers actually work?
2. Do humidifiers actually change the humidity?
One of the key researchers on humidifiers asked a brilliant question - do indoor humidifiers actually increase humidity? (18) This Colombia University researcher published the results of this study just last year. He explained that dry air can increase the risk of viral infections (he too had done a study showing this) (19).
Fine, we agree with this from our research too. He monitored indoor temperature and humidity in 34 apartments in New York City during winter 2014-2015. He found that ownership of a humidifier was not associated with higher indoor humidity levels. Game over.
So humidity matters. But not in the way that many people think. The relationship between humidity and disease is u-shaped. As a generalization, high and low humidity are not good for us. Midrange humidity is best. (That is so often the way of this world). That being said, practically speaking, none of this amounts to a 'hill of beans', as indoor humidifiers do not alter humidity anyway.
What a fuss about nothing.