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Evidence Brief: Humidifiers in health care 1

Evidence Brief: Humidifier use in health care

Key messages

 Improperly maintained cool mist

humidifiers can release aerosols

containing dissolved minerals and

opportunistic pathogens into the air.

Therefore, many public health and

health care institutions restrict their use.

 The decision on use of bedside

humidifiers for patients should consider

the type of humidifier, and cleaning and

maintenance of the humidifier.

Recommendations by other jurisdictions

may also be useful in decision-making.

April 2017

Issue and Research Question

Portable bedside humidifiers are used to

increase moisture levels in the air. The use of

humidifiers has benefits and drawbacks.

Humidifiers are used to relieve respiratory

symptoms such as shortness of breath

associated with bronchitis.

The moisture

supplied by the humidifier is also used to

alleviate dry skin

and help cold sufferers feel

better by reducing dryness and irritation to the

nose and throat.

Using humidifiers to keep the

relative humidity in the comfort range of 40%–

60% has been shown to decrease survival of

influenza viruses.

A humidity level above 60%

should be avoided because it can promote the

growth of microorganisms, including mould.

Humidifiers that atomize water to generate

micro-sized droplets in the air (referred to as an

aerosol) can potentially distribute any microbial

contaminants present in that water or in

biofilms that develop on humidifier tanks,

reservoirs, and spouts.

1,5

Contaminated room

humidifiers can generate high exposures

and

have been linked to both health care-acquired

and community-acquired infections.

6-9

Given

the reported benefits versus the potential for

respiratory disorders, the appropriateness of

allowing bedside humidifiers for patients and

residents in institutions has been a topic of

debate. The following evidence brief was

prepared to inform decisions on whether to

allow bedside humidifiers for patients.

Evidence Brief: Humidifiers in health care 2

Types of bedside humidifiers

 Ultrasonic humidifiers: These

produce a micro-sized aerosol when

a reservoir of water is vibrated at

ultrasonic frequency.

 Impeller humidifiers: A motor-

driven impeller (spinning disc) draws

up water and forces it through a

screen, breaking up the water into a

micro-sized aerosol.

 Evaporative: These use a reservoir

of water from which a wick filter

draws moisture A fan blows air over

the filter, creating moisture (via

evaporation of water) and blows the

moisture into the air.

 Steam humidifiers: These boil water

to emit water vapour (steam) into

the air. However, it is generally

cooled to prevent scalding due to

the high temperature of the steam

(>100°C).

 Because ultrasonic, impeller and

evaporative humidifiers do not heat

the water, they are commonly

referred to as “cool mist”

humidifiers by the manufacturers.

Methods

A review of the literature was executed using

OVID Medline, Academic Search Premier,

Environment Complete and Scopus databases,

Google, and Google Scholar. Search terms

related to humidifiers, cleaning, disinfectant,

and biocide were combined in the search

queries. Limits on the language of publication

(English) and date of publication (1980–present)

were applied to the searches. To complement

this review, references of included papers were

checked for other relevant papers.

Main Findings

Findings from the literature are summarized

below:

 Because several opportunistic pathogens

(such as Legionella, Pseudomonas,

nontuberculous mycobacteria and

Acinetobacter) are found in the natural

environment, they can be introduced into a

building’s plumbing system via the public

water supply.

Given the right conditions,

these opportunistic pathogens can then

colonize components of the building’s water

supply and contaminate the potable water.

If the building tap water is then used in an

aerosol producing humidifier (ultrasonic or

impeller unit), the pathogens can potentially

be aerosolized into indoor air and pose a

health risk. This is particularly true if the

humidifier is not routinely cleaned and

disinfected and the water in the humidifier’s

reservoir is allowed to stagnate.

 Improperly maintained humidifiers have

been associated with increased risk of illness

such as humidifier fever

1,3

and Legionnaires’

disease

 There are a number of different humidifier

designs (see callout box above), each with

different levels of risk. Steam humidifiers

(where water is boiled to create water

vapour that is released into the airstream)

represent a low level of risk of exposure to

pathogens, because steam is unlikely to

produce aerosols containing viable

microorganism.

Similarly, evaporative

humidifiers do not pose a significant health

Evidence Brief: Humidifiers in health care 3

risk, as they do not generate aerosols.

Alternatively, because ultrasonic and

impeller humidifiers can produce aerosols

possibly contaminated with pathogenic

bacteria, the risk potential for disease

transmission is higher, especially in

situations where there is a vulnerable person

in close proximity

11,12

The size of the

particles generated by ultrasonic and

impeller humidifiers is small enough to be

inhaled into the lungs. Highsmith et al. noted

that fine particles (< 2.5-µm in diameter)

constitute >90% but only 60-75% of the

particles generated by ultrasonic and

impeller humidifiers respectively

 A review of the literature indicates that most

disease transmission associated with

portable humidifiers is due to aerosol

producing humidifiers, i.e., ultrasonic and

impeller units that were not properly

maintained, cleaned or disinfected.

6,7,14-19

 Once they become contaminated, bedside

humidifiers are very difficult to clean. They

can still be a source of exposure even if

cleaning is attempted and they are refilled

with sterile water.

This is especially true if

mineral deposits or biofilm are present in

the humidifier. Mineral deposits and biofilm

not only protect the microbes from

disinfection, but provide a source of

nutrients for them to proliferate.

 When it comes to the use and care of home-

use humidifiers, guidelines from government

agencies responsible for protecting the

general public suggest following the

manufacturer’s instructions.

22-24

However,

different manufacturers have different

requirements, and there has been at least

one case where following the

manufacturer’s instructions (which allowed

the use of tap water for an ultrasonic

humidifier) resulted in an outbreak of

Legionnaires’ disease in a hospital nursery.

 Depending on the geological location, tap

water can be “hard.” Hard water contains a

higher concentration of minerals such as

sodium, calcium and magnesium.

These

mineral particles can be aerosolized (as a

“white dust”), potentially causing adverse

reactions in susceptible people.

To avoid

this, manufacturers of ultrasonic and

impeller humidifiers and researchers often

recommend the use of distilled water (or

demineralized water).

13,25

Some ultrasonic

and impeller humidifiers can be equipped

with demineralizing filters, but these are

expensive and have to be changed

frequently. Steam vaporizer and evaporative

humidifiers are not expected to aerosolize

any minerals particles into the air.

 Fatal lung disease has been reported when

ultrasonic humidifiers were run with biocide

detergents in the water reservoir rather than

it being used to clean the reservoir.

26-28

Recommendations for bedside

humidifiers in health care

institutions

Various health care institutions and public

health agencies have made different

recommendations concerning the use of

bedside humidifiers:

 Current American National Standards

Institute/American Society of Heating,

Refrigerating, and Air-Conditioning

Engineers/American Society for Healthcare

Engineering (ANSI/ASHRAE/ASHE)

ventilation guidelines state that humidifiers

should be located within air handling units

Evidence Brief: Humidifiers in health care 4

or ductwork, and that steam humidification

should be used.

 The United States Department of Veterans

Affairs, Veterans Health Administration

requires that only steam be used for building

humidification purposes, and prohibits the

use of ultrasonic (cool mist) humidifiers.

 Current CDC environmental infection control

guidance states that “Reservoir-type

humidifiers are not allowed in health care

facilities as per AIA (American Institute of

Architects) guidelines and many state codes.

Cool mist humidifiers (ultrasonic and

impeller) should be avoided because they

can disseminate aerosols containing

allergens and microorganisms. Additionally,

the small, personal-use versions of this

equipment can be difficult to clean”.

31,32

 In its guideline for preventing health-care

associated pneumonia, the CDC warns

against the use of room-air humidifiers that

create aerosols unless they can be sterilized

or subjected to high-level disinfection at

least daily and filled only with sterile water.

They also recommend the use of sterile

water for rinsing after chemical disinfection.

Note: Distilled water may not be sterile.

There have been at least two cases where

distilled water used to fill humidifiers has led

to infections.

34,35

 In instances of nosocomial infections traced

to aerosol-generating equipment in patients'

rooms, the WHO notes that “substitution of

steam humidifiers for equipment that

provides humidification by the

aerosolization of unsterile water” has been

followed by cessation of further cases.

 The National Institutes of Health recently

conducted a study to determine the risk to

health of their humidifier policy.

The policy

allows patients to request bedside

humidifiers, but states that humidifiers

“must be filled with sterile water and that

the humidifier must be discarded after three

days of use.” In the study, ultrasonic

humidifiers were filled with sterile water and

the microbial content examined over five

days. The study found that contamination

appeared after just one day, and increased

over time. It concluded that aerosolization of

potential pathogens by this type of

humidifier may pose a risk, especially to

individuals whose immune systems are

suppressed. Based on the study’s findings,

the authors recommended against the use of

bedside in hospitals.

Discussion and conclusions

Humidifiers are used to help maintain

comfortable room humidity. There are four

types of humidifiers: evaporative, steam,

ultrasonic and impeller. Ultrasonic and impeller

humidifiers generally pose a greater risk than

evaporative and steam humidifiers because

evaporative and steam humidifiers are not

expected to aerosolize any mineral products

into the air. This is because they can release

aerosols containing dissolved minerals and

opportunistic pathogens into the air. Many

public health and health care institutions

restrict their use or allow their use only under

strict conditions. Some allow only the use of

steam for building humidification.

Implications for practice

Portable bedside humidifiers are used to

increase moisture levels in the air. The use of

some types of humidifiers may create a risk of

Evidence Brief: Humidifiers in health care 5

infection. The decision to allow bedside

humidifiers should consider:

 The type of bedside humidifier as steam

and evaporative-type represent a low level

of exposure to pathogens and dissolved

mineral particles whereas ultrasonic and

impeller humidifiers can be a source of

exposure to the dissolved and suspended

components, including pathogens, of the

water.

 Proper maintenance, cleaning or

disinfection of the humidifier in accordance

with manufacturers’ specifications.

 Other jurisdictions also provide

recommendations that may be considered

in decision-making.

Specifications and limitations of

Evidence Briefs

The purpose of this Evidence Brief is to

investigate a research question to help inform

decision making. The Evidence Brief presents

key findings based on a systematic search of the

best available evidence near the time of

publication, as well as systematic screening and

extraction of data from that evidence. It does

not report the same level of detail as a full

systematic review. Every attempt has been

made to incorporate the highest level of

evidence on the topic. There may be relevant

individual studies that are not included;

however, it is important to consider at the time

of use of this Evidence Brief whether individual

studies would alter the conclusions drawn from

the document.

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Author

Dru Sahai, MSc(A), ROH, CPHI(C), CRSP

Environmental Science Specialist, Environmental and Occupational Health, Public Health Ontario

Reviewer

Ray Copes, MD, MSc

Chief, Environmental and Occupational Health, Public Health Ontario

Acknowledgements

Isabelle Langman, Program Infection Prevention Control Specialist, Public Health Ontario

Sam MacFarlane, Program Infection Prevention Control Specialist, Public Health Ontario

Citation

Ontario Agency for Health Protection and Promotion (Public Health Ontario), Sahai D. Evidence Brief:

Humidifier use in health care. Toronto, ON: Queen’s Printer for Ontario; 2017.

ISBN: 978-1-4606-8773-4

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