White paper

Humidity control in indoor environments:

Implications for health,comfort, and productintegrity

Executive summary

Indoor air quality plays a pivotal role in shaping ourhealth, comfort, and overall well-being. Among thevarious factors that contribute to a healthy indoor
environment, relative humidity emerges as a criticalparameter that often goes underestimated. The
level of moisture in the air, commonly referred toas relative humidity (RH), has profound effects onhuman health, material performance and how
electronics function.
Humidity control impacts diverse indoor environments. From residential spaces to industrial facilities, maintaining optimal humidity levels is essential

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for fostering a healthy atmosphere. There arevarious methods employed to regulate humidity,
from evaporative (adiabatic) to steam (isothermal)
humidification. There are also specific applicationsof humidity control in different settings, such as
homes, offices, industrial processes, and specialized
environments like healthcare facilities, libraries,
and museums.
Humidity control is not merely a luxury but a necessity for promoting health, preserving materials, andensuring optimal functioning of equipment in indoor
environments.

Relationship between temperatureand humidity
For reference the following definitions have been
adopted in this document:
Absolute Humidity (AH) – Alternatively called
the Humidity Ratio or Specific Humidity is the
moisture content in gr/kg (dry air)Relative Humidity (RH) – The ratio (in %) between
the actual quantity of water vapor in the air andthe maximum quantity of water vapor that the air
can contain at a certain temperature. At 100%
RH, air is saturated and can hold no additional
moisture. Condensation will then occur if thetemperature is reduced
AH is a useful measure for converting RH levels
between environments at different temperatures,such as indoor and outdoor conditions. If there areno additional sources of moisture or humidity in
an indoor environment with outdoor ventilation,then changes in outdoor RH will influence indoor
RH levels.

1 kg air
1 kg air

100% RH
6g

1 kg air

80% RH6g

23% RH
6g

6.5°C
10°C
Figure 1: Visual representation of the relationshipbetween air mass, temperature, AH and RH%

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30°C

The relationship between temperature and humidity
is demonstrated in the following illustration. In all
three cases, the Absolute Humidity is constant,since the amount of water is six gr/kg (dry air) and
air weight is one kg.
As temperature increases the air volume expands,
and so the relationship between the Relative
Humidity and the occupied space changes.
When calculating indoor humidity levels, we needto take into account the prevailing outdoor levels of
RH and AH. If we assume that outdoor air is admitted into an empty indoor space and heated with nosignificant moisture loads present (e.g. no moistureis introduced from ingression, building materials,
combustion or occupants), then we can establish
a direct relationship between indoor and outdoor
humidity levels at different temperatures.

To demonstrate this, the table below shows indoor
humidity levels that would occur in an indoor spacedescribed in the paragraph above, based on theoutdoor levels reported in London on 12th–13th
March 2020.

By using a psychrometric chart, we can easily
visualise the effect on RH% levels when altering
air temperature at specific AH levels. By using themeasurement value in Table 1 for AH at 09:00 on
March 12th (4 g/kg), the connection between RH
levels at 4 and 20°C becomes apparent.

Based upon the outdoor conditions shown in Table 1,at an indoor temperature of 20°C the indoor RH%
level varies between 24 to 31% RH. If the indoor
temperature increases to 24°C, the indoor RH% levelvaries between 19 to 24% RH.

Time

9:00

12:00

15:00

18:00

21:00

00:00

03:00

06:00

9:00

Outdoor temp (C)

8°

9°

9°

7°

7°

6°

7°

6°

8°

Outdoor RH%

60

49

52

67

68

77

70

73

62

Air pressure (hPa)

1,011

1,011

1,010

1,011

1,011

1,013

1,014

1,016

1,017

Outdoor AH (g/kg)

4.00

3.50

3.71

4.18

4.24

4.47

4.35

4.23

4.11

Indoor RH% @ 20°C

28

24

26

29

29

31

30

30

28

Indoor RH% @ 24°C

22

19

20

23

23

24

24

23

22

Table 1: Climate conditions for London, UK on 12th–13th March 2020. Source: Met Office, UK

%

%

%

40

30

20

19
15%

%

%
50

20

60

%

%
70

%

%
90

100

101.32 kPa

30
80

25

Psychrometric chart

18
17

60
20

15

50

14
13
12

8%

10

%
10

40

th

al

,
py

20

10

u
-b
et
W-5

0

-10
-20

-20

-15

-15

lb

m
te

pe

11

15

30

°C
e, 10
ur
t
ra

9
8
6%

En

g
/k
kJ

7
6

Indoor conditions
20°C, 28%RH

5

5

4%

Humidity ratio, g/kg (dry air)

16

4

0

3

Outdoor conditions
8°C, 60%RH

2% R

H
2
1

-10

-5

5

0

0
10

15

20

25

30

35

40

45

50

55

60

Dry-bulb temperature, °C
5

4

10

20

30

40

50

60

Figure 2: Psychrometric chart
showing 8 and 20°C RH% conditions
along the AH 4 g/kg line.

Importance of Relative Humidity:
Human comfort and health: RH influences how
the human body perceives temperature. At
higher humidity levels, the body’s ability todissipate heat through perspiration is hindered,leading to a feeling of discomfort. Conversely,
low humidity can result in dry skin, irritated eyes,
and respiratory discomfort. Maintaining an
optimal RH level contributes to a comfortable
and healthy indoor environment.
Respiratory health: Dry indoor air can exacerbate
respiratory conditions, such as allergies, asthma,and cold symptoms. Inadequate humidity levels
can dry out the mucous membranes in the
respiratory tract, making individuals more
susceptible to infections and allergies.
Viral transmission: Research suggests thatindoor environments with proper humidity levels
may play a role in mitigating the spread ofviruses. While humidity alone cannot prevent
virus transmission, maintaining an appropriate
range (around 40-60% RH) may help reduce theviability and transmission of certain viruses.

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Effects of too high or too low humidity:
High humidity: Excessive humidity can createa breeding ground for mold, mildew, and dust
mites, triggering allergies and respiratory issues.
It can also compromise building materials,
leading to decay, peeling paint, and structural
damage.
Low humidity: In environments with low humidity,
moisture is drawn from surfaces and livingorganisms, including the human body. This canlead to dry skin, irritated eyes, and increased
susceptibility to infections. Low humidity can also
cause materials like wood, textiles, and paper tobecome brittle and crack.

Health and well-being
General well-being:
Humidity levels significantly impact skin health andgeneral well-being. Low humidity can cause skindryness, itching, and discomfort. Adequate humidity
helps maintain the skin’s natural moisture balance,
promoting overall comfort. Furthermore, properly
humidified indoor spaces can alleviate symptoms
related to dry eyes and nasal passages, enhancing
the overall quality of life.
Respiratory health and humidity:
A well-maintained indoor humidity environment
plays a vital role in supporting respiratory health.
Dry air can irritate the respiratory tract, potentially worsening conditions like asthma and allergies. Mucous membranes in the respiratory system
require a certain level of moisture to function optimally, ensuring that they effectively trap and expel
airborne particles, allergens, and pathogens.
Occupational illnesses and humidity-related risks:
Occupational settings that expose workers to low
humidity conditions can lead to a range of healthissues. Dry air can cause dry eyes, throat irritation,and skin discomfort among workers. Additionally,
workers in environments with inadequate humidity
levels may experience a higher frequency of
respiratory infections due to compromised
mucous membranes.
Influence of humidity on viral transmission:
Viruses that cause respiratory illnesses, like the flu
and coronaviruses, are mainly spread through tiny
viral particles released when people cough, sneeze,
talk, or even breathe. These particles come in various sizes: large droplets (>50 μm) fall quickly to theground, particles in the 10–50 μm range settle in a
few minutes, and small particles (<10 μm) can float
in the air for hours and get deep into our lungs orland on surfaces.

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The time these particles stay in the air depends onairflow and humidity. For instance, when indoor
humidity is around 45-50%, the infectivity of flu
particles drops significantly. Humidity also affects
coronaviruses but studying them is presents inherent risks and personnel must work under biosafetylevel 3 laboratory containment conditions. But theuse of surrogate coronaviruses, such as TGEV and
MHV, has the potential to overcome these challenges and expand the available data on coronavirus
survival on surfaces.
At lower humidity (20%) and higher humidity (80%),these surrogate coronaviruses survived better thanat moderate humidity (50%). Warmer temperatures
also helped inactivate them faster than cooler ones.
It is possible to reduce the transmission and longevity of harmful viruses by controlling indoor RH
levels. This can be done by using humidity control
solutions which facilitate precise RH control forindoor environments. This humidity control is usefulat all times, but particularly in typical ‘flu season,’where outdoor temperatures and Absolute Humidity
tend to be lower.
Research referenced in this document suggeststhat maintaining an indoor RH% between 40 to 50%
yields the maximum protective effect from aerosolized and settled virus particles. By maintaining
these levels, it is possible to contribute to a healthier, safer environment.

Energy and environmental
considerations
Humidity plays a pivotal role in how we perceive
temperature. Even though the dry-bulb temperature remains constant, higher humidity levels canmake the air feel warmer due to reduced evaporation from the skin. Conversely, lower humidity levels
can lead to the perception of cooler air. This phenomenon has significant implications for thermal
comfort and can impact energy consumption
in heating and cooling systems.
Efficient humidity control can contribute to energy
savings. By optimizing humidity levels, HVAC systems can operate more effectively and consumeless energy. In colder climates, maintaining proper

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humidity levels can reduce the need for higher
heating temperatures. While in warmer climateseffective humidity control can help occupants feel
comfortable at slightly higher temperatures, thereby
reducing air conditioning demands.
Furthermore, proper humidity regulation can resultin improved well-being and reduced sick leave.
Maintaining optimal indoor humidity levels can prevent the proliferation of mold, bacteria, and viruses
that thrive in excessively damp or dry environments.
This can lead to a healthier indoor environment,
reducing the risk of respiratory illnesses and allergies among occupantsHumidity’s impact on materials

Humidity significantly influences various materials
and electronic systems. Static electricity generation is a well-known consequence of dry conditions.
When humidity is low, materials like plastics, paper,
hair, and textiles tend to generate static electricity
through friction. This can lead to uncomfortableshocks, but it also poses risks to electronic components and sensitive equipment.
Materials used in construction are also deeply
affected by humidity levels. Wood, textiles, andbuilding materials like concrete are all hygroscopic,
meaning they can absorb, and release moisture
based on the surrounding humidity.

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Improper humidity levels can lead to warping,cracking, and even structural damage. Therefore,
maintaining optimal humidity is critical to preserving
material integrity and longevity.
Electronics are highly sensitive to humidity as well.
High humidity can lead to condensation on electronic components, potentially causing short circuits and corrosion. Conversely, very low humidity
can result in the buildup of static electricity, which
poses a risk of damaging sensitive electronic circuits.
Controlling humidity in data centers and electronics
manufacturing facilities is essential to ensure reliable performance and prevent costly failures.

Humidification methods
Several methods are employed to control humidity
levels in various settings:
Evaporative (adiabatic) humidification andcooling systems use a process that adds watermolecules to the air to increase relative humidity
and lower the temperature. Depending on yourapplication, increased humidity or a lower
temperature are the desired outcomes.
Evaporative humidifiers are commonly used in air
handling units, data centers, and larger facilities
with cooling needs.

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Steam (isothermal) humidification systems raise
humidity levels to restore optimal indoor climate.
Using boiling water to produce the steam, theseultra-hygienic systems are free from harmful
substances. This method is versatile and suitablefor various scales and control systems, commonlyused in industries and commercial premises.
These humidification methods find applications invarious contexts, from household humidification andoffice spaces to industrial processes, plantations,and greenhouses. Selecting the appropriate method
depends on factors such as the size of the area,
required humidity levels, energy efficiency considerations, and specific environmental conditions.

Humidity control in indoor
environments
Humidity control varies across different indoor environments, each with its unique considerations andrequirements:
Office spaces and commercial premises:
Humidification in office environments is typicallynot standardized. Some landlords incorporate
humidity control into climate systems, whileothers leave it up to employees to arrange forstandalone humidifiers. As we spend
considerable time in these spaces, ensuring
optimal humidity can enhance comfort,
productivity, and overall well-being.
Industrial applications and manufacturing
processes: Industries often have specific humidity
requirements to ensure product quality,
equipment performance, and safety. Frompreventing wood warping in the construction
sector to maintaining the freshness of food instorage facilities, humidity control is integral to
many manufacturing processes.

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Server rooms and data centers: Data centershouse vast quantities of electronics and requireprecise humidity control. Maintaining humidity
levels between 40% and 65% RH is crucial toprevent static discharges and ensure thereliability of sensitive components. Evaporative
humidification can be particularly valuable inthese environments, even during colder seasons.
Household humidification: In households,
humidification is often managed throughstandalone devices like electric steam humidifiers
or open water containers. Many individuals use
these methods to alleviate discomfort caused bydry air, but there’s room for awareness andimprovement. Integrating humidification systems
into HVAC setups could ensure consistent andoptimal humidity levels throughout homes.

Humidity control in specialized
environments
Humidity control takes on a new dimension in specialized environments with distinct needs:
Health care facilities: Hospitals and healthcare
centers prioritize air quality due to sensitive
patients and surgical procedures. Optimal
humidity levels (25-50% RH) contribute topatient well-being and infection prevention.
Maintaining these levels requires specialized
systems to ensure optimal care and recovery
environments.
Libraries and archives: Libraries and archives
house valuable materials vulnerable to humidity
fluctuations. Precise control (12-18°C and
30-40% RH) preserves the integrity of books,
documents, and other artifacts. Proper climate
control prevents mold, degradation, and otherforms of damage that can compromise thelongevity of historical collections.

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Museums and artifact preservation: Museums
play a critical role in safeguarding cultural
heritage. Humidity control (20-22°C and40-60% RH) is vital for protecting paintings,
sculptures, textiles, and other artifacts. This
practice prevents degradation, cracking, andfading, ensuring the preservation of art for
generations to come.
Food production and storage: The food industry
relies on optimal humidity levels to maintainproduct quality and safety. From preventing
spoilage in cold storage to ensuring consistentproduction conditions, controlling humidity(40-95% RH) is essential to preserve food quality
and extend shelf life.

Conclusion
Humidity control plays a multifaceted role acrossdiverse environments, impacting everything fromcomfort and health to product quality and historicalpreservation. The significance of maintaining optimal humidity levels cannot be overstated. By raisingawareness and implementing effective humidity
control strategies, we can create more comfortable,
productive, and sustainable environments for individuals and industries alike.
As we move forward, future trends and innovations
in humidity control will likely focus on enhancedenergy efficiency, integration with smart building
technologies, and the development of eco-friendly
solutions. By embracing these advancements,
we can continue to improve indoor environments,
bolstering well-being and efficiency for generations
to come.

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About Munters
Munters has more than 60 years of experience inhumidity control, with scalable solutions for all
types of industrial, commercial and public service
environments. We offer consultation, engineering
support, installation and life cycle management for
our flexible range of products, with a global presence that keeps us close to our customers.
The right climate to change the future
Munters is a global leader in energy-efficient andsustainable air treatment solutions. Using innovativetechnologies, Munters creates the perfect climate
for demanding industrial applications and agriculture sectors. Munters has been defining the futureof air treatment since its founding in 1955, and has
been listed on Nasdaq Stockholm since 2017.

Munters is a global leader in energy-efficient air treatment
and climate solutions. Using innovative technologies, Munterscreates the perfect climate for customers in a wide range ofindustries.
Munters has been defining the future of air treatment
since 1955. Today, around 4,000 employees carry out
manufacturing and sales in more than 30 countries.

Humidification-WP-EN-202310

For more information, please visit www.munters.com

Munters reserves the right to make alterations to specifications, quantities, etc., for production or other reasons,subsequent to publication. © Munters AB, 2023