Relative Humidity

Relative humidity (Specific humidity) is the ratio of the amount of water vapor currently present in the air to the maximum amount of water vapor the air can hold at the same temperature. This ratio is typically expressed as a percentage (%). The capacity of air to hold water vapor is primarily a function of air temperature; warm air can contain or hold more moisture than cold air. When air reaches its maximum moisture capacity, it is said to be saturated, and at this point, relative humidity is measured at 100%.

Relative humidity is an environmental parameter studied in many fields including geography, climatology, human health, and the preservation of cultural heritage.

Conceptual Distinctions and Terminology

In geography education, concepts related to humidity are frequently confused by students due to their abstract nature. The fundamental concepts are absolute humidity, absolute humidity capacity, and relative humidity ratio.

• Absolute Humidity: The weight in grams of water vapor in 1 m³ of air volume, indicating the actual amount of water vapor present in the atmosphere.

• Absolute Humidity Capacity: The maximum amount of water vapor that air can hold at a specific temperature. This concept is also referred to as “saturation amount” or “maximum humidity”.

• Relative Humidity Ratio: The ratio of the absolute humidity amount to the absolute humidity capacity (or saturation amount).

Incorrect usage of these concepts is sometimes found in textbooks. For example, “absolute humidity amount” and “absolute humidity capacity” are often used interchangeably. The statement “as temperature increases, absolute humidity increases” is not always true; what increases is the absolute humidity capacity. The actual amount of absolute humidity depends on the environment where the air is located, that is, its proximity to moisture sources.

Similarly, the statement “as temperature increases, relative humidity decreases” is not an absolute rule. With increasing temperature (which increases capacity), if moisture sources are sufficient and the absolute humidity amount also increases, the relative humidity ratio may remain high or even rise. Therefore, it must be emphasized that relative humidity is not a “quantity” but a “ratio”.

Effects on Health

Relative humidity (RH) is an environmental parameter that affects human health in both indoor and outdoor environments.

Optimal Range and Health Risks

The optimal relative humidity range for health is considered to be 40–60%. Conditions below or above this range can create conditions conducive to the exacerbation of respiratory diseases and the transmission of infections.

• Low Relative Humidity: RH below 40% can cause dryness and irritation in the respiratory tract and skin, making individuals more susceptible to infections. Low humidity also promotes indoor ozone formation.

• High Relative Humidity: RH above 60% creates a moist environment that encourages the growth of harmful microorganisms such as mold, bacteria, and viruses. It can also increase the emission of chemicals such as formaldehyde from building and furniture materials.

Mucosa, Mucociliary Clearance, and Immunity

The primary effect of relative humidity is to alter the rheological properties and osmolarity of mucus, thereby affecting mucociliary clearance (the self-cleaning mechanism of the respiratory tract).

Inhalation of cold air in the upper respiratory tract increases mucus viscosity and reduces ciliary beating, thereby increasing susceptibility to viral infections. In the lower respiratory tract, excessive or thickened mucus can facilitate bacterial adhesion. This disruption of the mucus layer can compromise the structure of tight junctions that maintain epithelial integrity, increasing the permeability of the mucosal barrier to contaminants, allergens, and viruses. Damaged epithelial cells may release mediators such as alarmins (IL33, TSLP, IL25), which can initiate an inflammatory cascade.

Effects on Pathogens and Allergens

Relative humidity levels directly influence the growth of pathogens and allergens.

• Viruses: The aerosol stability of the influenza virus is highest at low RH levels such as 20–40%. Generally, viruses such as influenza, parainfluenza, and adenovirus have been observed to increase in concentration at RH below 50% and above 70%.

• Bacteria: The growth of certain bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa increases at RH below 30% and above 60%.

• Allergens: Excess indoor humidity is associated with the growth of allergens such as mold and house dust mites. It has been found that mites are absent at RH levels below 50% during winter months, while indoor fungi (mold) become widespread above 60% RH.

Museology and Artifact Conservation

Relative humidity is a critical parameter in the conservation of museum collections and cultural heritage.

Evolution of Museum Standards

Museum specifications for relative humidity became rigid in the 1960s and 1970s. In 1929, engineer MacIntyre, without scientific basis, recommended 55–60% RH for the National Gallery in London based on London’s climate and technical feasibility. Later, it was determined that the average annual moisture content in the gallery’s wooden artifacts corresponded to 58% RH. Over time, values such as 50% or 55% RH, along with narrow fluctuation ranges of ±2% or ±5%, became standard. However, these standards were based not on the needs of the artifacts but on the technical feasibility of the time. The 50% RH value emphasized mechanical phenomena while neglecting chemical ones. The installation, maintenance, and damage caused to buildings in cold climates due to humidification were costly.

Degradation Effects on Materials

The effects of relative humidity on artifacts are examined in four main categories: dampness, critical thresholds, RH above 0%, and RH fluctuations.

• High Humidity (Damp): All forms of mechanical, biological (mold), and chemical degradation increase sharply above 75% RH. Practical mold risk typically begins above 75% RH (e.g., 80% for dirty surfaces or leather, 90% for clean paper). RH levels of 90% lasting no more than one or two days are tolerable from a mold perspective. Inorganic degradation, such as corrosion on metal surfaces and the deliquescence of common salts like NaCl at 76% RH, also accelerates around 75% RH.

• Chemical Degradation (RH > 0%): Chemical degradation such as hydrolysis continues even at low humidity. Paper strength loss follows a power law proportional to the 1.35 power of RH. Maintaining a constant 50% RH throughout the year in cold climates, thereby eliminating the slowing effect of low winter humidity on acidic paper, has approximately doubled the rate of chemical degradation.

• Mechanical Damage (Fluctuations): This is the primary risk factor for rigid and constrained organic materials such as wooden panels, gesso, glue, and bone. A sudden drop of approximately -25% to -50% RH in a single cycle is typically required to cause fracture. According to fatigue models, smaller fluctuations below this critical threshold (e.g., ±10%) nearly eliminate damage.

• Material Behavior at Low and High RH: Low RH causes materials such as animal glue and gesso to become strong but glass-like (brittle), increasing the risk of cracking by extending stress relaxation times from days to months. High RH (above 80%) weakens these same adhesives, making them rubbery; this can cause veneers to slip or warp.

However, many objects that are flexible (e.g., textiles) or mobile (e.g., well-designed wooden joinery) are less affected by these fluctuations.

Climate Change and Atmospheric Models

In the context of climate change, relative humidity (R) is analyzed to understand climate feedbacks from water vapor and clouds.

Trends in General Circulation Models (GCMs)

General Circulation Models (GCMs) predict a characteristic pattern of change in relative humidity as the climate warms. Although models suggest that the global water vapor feedback is close to maintaining a fixed relative humidity distribution independent of climate, this does not mean that relative humidity remains unchanged everywhere.

Model projections for R include an increase around the tropopause, a decrease in the tropical upper troposphere, and a decrease in mid-latitudes. This pattern of change resembles the cloud cover changes simulated in the same GCMs, confirming that cloud changes in models are controlled by R.

The magnitude of R changes tends to be related to the horizontal resolution of the model. Outside tropical regions, when GCM resolutions exceed T85 (approximately 2°), results approach current observations and show convergence.

The “Shift” Hypothesis and Its Limitations

A simple hypothesis used to explain the predicted R change pattern is that the current R distribution “shifts” upward with the warming tropopause and poleward with the zonal jets. Indeed, regions where R decreases (δR < 0) roughly correspond to areas where R currently increases poleward or upward.

However, analysis of models shows that this “shift” explanation alone is insufficient.

• The subtropical (mid-latitude) drying trend is approximately three times larger than what could be attributed to the poleward shift of subtropical features such as the Hadley cell expansion.

• The calculated rate of rise in the tropical upper (TU) region (13 hPa K⁻¹) is about four times higher than the average tropopause rise rate in models (~3 hPa K⁻¹).

• Additionally, in most models, the subtropical R minima (regions of lowest R) are observed to deepen; this cannot be explained by a simple shift alone.

An alternative explanation is that R depends on non-local “last-saturation” temperature changes. The R value of air depends on the difference between its current temperature and the temperature at which it last reached saturation (LS). If the last-saturation point occurs in distant regions experiencing different temperature changes, the R value will change accordingly.