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According to this report

"Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens"

Similarly, a NY Times article published in 1988 states that:

"Anyone who is allergic to molds should not use cool-mist vaporizers or ultrasonic humidifiers, since both spray water into the air, Dr. Burge said. Cool-mist vaporizers pose a greater hazard since they can disperse live organisms into the air. Studies have linked the use of cool-mist vaporizers in hospital rooms with severe respiratory infections."

I understand this to mean that humidifiers increase risk of infection because they spur mold, allergens and other "live organisms" into the air - thereby increasing the amount of allergens and "toxic" substances we inhale.

1) Is there any well researched evidence to back up the claim that humidifiers spur "mold", "allergens" and "live organisms" into the air and increase respiratory infections?

2) The NY Times article states that studies have linked humidifiers to serious infections "in hospitals". Does this apply to healthy individuals as well?

3) Finally, are all humidifiers created equal in this regard?

P.S

For those of you who are unaware there are several types of humidifiers:

Steam vaporizers - uses electricity to generate steam

Ultrasonic humidifiers - oscillates 1.7 million times a second to break water into a fine mist

Evaporators - produces humidity by blowing air past evaporating water

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    I think this needs to be rephrased a bit, to focus down on a single claim. We don't need the peripheral information about your intent to purchase, and the title should directly reflect the claim that you want to question, rather than a general request for information. If a general request for pertinent information is truly what you want, though, you might be better served to ask on the health stack exchange, rather than here at skeptics.
    – Ben Barden
    Nov 20, 2018 at 21:32
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    It's unclear how a "cool mist" humidifier would "disseminate aerosols" worse than your standard evaporative one. If the water's contaminated then the water's contaminated.
    – Daniel R Hicks
    Nov 21, 2018 at 1:31
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    This question is still a bit confusing. The first link you provide contains well-researched evidence to back up the claim that relative humidity affects the load of toxic aerosols in the air. Even if the humidifier doesn't directly spray mold in the air, it promotes the growth of mold.
    – Oddthinking ♦
    Nov 21, 2018 at 2:15
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    The main point here is, if the water is contaminated -- from the beginning or because the humidifier wasn't cleaned properly and in turn contaminated the water -- then turning that contaminated water into aerosol form of course disseminates the contamination.
    – DevSolar
    Nov 21, 2018 at 9:48
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    @Oddthinking There are many low cost humidifiers available today that have inbuilt sensors that detect amount of humidity in the air and adjust output accordingly. So it is relatively simple to keep a uniform and static level of humidity.
    – S.O.S
    Nov 21, 2018 at 16:33

1 Answer 1

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In 2008, three newborns died as part of an outbreak of Legionnaires' disease in a hospital.

The source was found to be a cold mist ultrasonic humidifier.

A case study described the investigation that lead to this conclusion:

It was determined that the neonates were infected while in the nursery at the private hospital by aerosol produced by a recently installed cold-mist humidifier that was filled with contaminated water.

[...]

Use of humidifiers in nursery units must be avoided as the risk of disseminating Legionella in neonates is very high.

Of course, we aren't all as succeptable as newborn babies, but there is still a risk. A 2017 case study, Community-Acquired Cavitary Pseudomonas Pneumonia Linked to Use of a Home Humidifier explains how a previously healthy 30 year old male succumbed to pneumonia he caught from a home humidifier unit:

We asked the patient’s family to bring his ultrasonic cool-mist humidifier from home. It was noted to have a thin gray-green film coating the plastic covering as well as standing water in the basin [...] The patient was unsure when the unit had last been cleaned.

I think cases like this explain the recommendations in the quotes given.

Outside of hospitals, there are studies like this one from 1995, Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxins, and Allergens.

While it didn't track the absolute incidence of diseases, it did examine the water found in home units:

Clinically insignificant as well as overt or potentially pathogenic microorganisms were found to colonize the reservoirs of all types of humidifiers [...] Overt pathogens isolated from humidifiers in homes included Legionella and a pathogenic Acanthamoeba.

They also examined whether these microorganisms were put in the air:

only cool mist and ultrasonic units readily aerosolized bacteria and endotoxin. Only cool mist units emitted hydrophobic fungal spores

This doesn't prove that home humidifiers are often contaminated, that the microorganisms in the air will likely infect you or that there aren't also health benefits to humidifiers that outweigh the risks.

However, I think it supports the actual claims made:

  • cool mist humidifiers can disseminate contaminated aerosols
  • cool-mist humidifiers can disperse live organisms into the air.
  • case studies have linked the use of cool-mist vaporizers in hospital rooms with severe respiratory infections.
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    That covers the "Cool-mist vaporizers pose a greater hazard since they can disperse live organisms into the air. Studies have linked the use of cool-mist vaporizers in hospital rooms with severe respiratory infections." but it doesn't talk about the allergens or the deal with mold.
    – Ben Barden
    Nov 21, 2018 at 15:16
  • Hmmm, I always thought legionnaire's was an STD. Learn something new every day.
    – user11643
    Nov 22, 2018 at 16:32
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    Although I presume the neonates were born healthy (since the report does not state otherwise), the study only covers a tiny spectrum of overall population since there's reason to believe newborns are more sensitive to this kind of thing. It does not tell us anything about babies (3+ months) or toddlers. Also, the study says that the humidifier was filled with "contaminated" water without going into specific details. Is tap water normally contaminated? If not then this study is the exception, not the rule.
    – S.O.S
    Nov 22, 2018 at 16:35
  • Isn't case study basically a kind phrase for anecdote? This isn't particularly strong evidence that humidifiers increase respiratory infection risk.
    – user11643
    Nov 22, 2018 at 16:38
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    I considered adding Assessing Inhalation Exposures Associated with Contamination Events in Water Distribution Systems which tried to model the dosages that humidifers might cause, but I didn't understand it well enough to be comfortable.
    – Oddthinking ♦
    Nov 23, 2018 at 6:47

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