When indoor air humidity increases, it produces adverse effect on human health – so humidity is an important parameter both in the industrial and residential environment. Moreover, exceptional high value of moisture in indoor air can affect Indoor Air Quality (IAQ) in indoor environments. Further to this, causation of perceived sensory reactions in eyes and respiratory system, among top-two reported symptoms, continues to be a puzzle to solve, despite several identified risk factors that influence the development of eye symptoms have been identified; the risks of symptoms in the upper airways remain largely unexplained. Furthermore, there is an increasing recognition of the impact of humidity, e.g. on virus survival, growth and transmission as well as sleep quality, regarding derivation of a safe limit for indoor air humidity. Thus, there is a need to control the impact of indoor air humidity on associated health effects – as opposed to the well-known problems associated with moisture-damaged buildings. The relationship between health, indoor air humidity and pollution is complex and remains a challenge. Thus, the focus of this overview is effects in the public domain of perceived IAQ, sensory irritation in eyes and airways, work performance, infection by virus, sleep quality, and the voice.
HVAC (Heating, Ventilation and Air Conditioning) systems are used to create a human thermal comfort inside residential and industrial buildings. However, they contribute to a large portion of overall final energy consumption: for example, around 50% of building’s final energy consumption and 20% of total energy consumption in the United States. This places a strain on the electricity network – as well as indirectly contributes to environmental problems such as ozone-layer depletion, air pollution and climate change.
Desiccant-based air-conditioning systems can reduce these negative effects, because they can be driven by waste heat, such as the heat discharged from distributed power generation, various co-generators and solar thermal collectors. As such, they require a much smaller amount of electricity to operate. In addition, they do not use chlorofluorocarbon refrigerants, and so do not harm the environment. It is found that the desiccant based dehumidification and cooling technology is still in the early stage of development, since many of the installed systems did not achieve the expected energy savings. Hence, further effort is required to improve the overall efficiency.
The desiccant component of the desiccant-based air-conditioning systems maybe based around a liquid-desiccant heat exchanger or a solid-desiccant rotary wheel. Solid-desiccant systems have several advantages over liquid-desiccant systems. These include avoiding potential desiccant loss and contamination of the building supply air due to the carryover of desiccant sorbent into the air streams, and avoiding corrosion issues that arise when using liquid desiccant. Disadvantages of solid-desiccant systems include the need for rotating air seals and the higher temperature of regeneration air.
In an air-conditioning system, the desiccant component performs latent cooling, while different combinations of direct evaporative coolers, indirect evaporative coolers, chiller coils or heat recovery devices perform the sensible cooling. Additional components may be present and several different operating cycles are possible. These include three different desiccant cooling cycles: the ventilation cycle, the recirculation cycle and the Dunkle cycle, which were analysed for evaluation of better system performance for different configurations of cycle. The results revealed that the Coefficient Of Performance (COP) could be improved further by strengthening the performance of the dehumidifier.
Two more desiccant cooling cycles were investigated, which were a simplified, advanced, solid desiccant cycle and a direct-indirect, evaporative cooling cycle with a COP higher than 2.0. The feasibility of combining desiccant cooling systems with conventional air-conditioning systems was analysed, which revealed that a desiccant driven by low temperature regeneration air was the key to improving the system’s energy performance. In addition to system studies, researchers have also investigated individual components, especially desiccant dehumidifiers. For example, modeling studies of the desiccant wheel used the effectiveness-NTU (Number of Transfer Units) method to predict the moisture removal performance of the desiccant wheel with a high accuracy. Structural studies of the wheel reduced the energy consumed by fans and blowers due to the decrease of drop pressure achieved by redesigning the traditional honeycomb matrix structure; the temperature of the regeneration air for desorption was also reduced.
Operating condition studies revealed that wheel rotation speed could significantly influence the performance of the wheel, and that the optimal rotational speed to maximise dehumidification effectiveness depends on the operating conditions. Optimisation studies to improve the dehumidification performance of the desiccant wheel adopted a model predictive control strategy to set a maximum moisture removal capacity for the desiccant wheel.
Studies of the heat source used to make regeneration air for the desiccant wheel showed that regeneration of air could be made using thermal energy from a micro-cogenerator with a temperature of less than 70°C, while involved heating regeneration air using a solar hot water system.
The above studies mainly focused on two aspects: (i) improving the efficiency of system (especially for the low temperature of regeneration air), and (ii) improving the dehumidification capacity of the desiccant wheel.
Recently, researchers have begun to focus on improving the dehumidification performance of the desiccant component by targeting an isothermal, rather than adiabatic, dehumidification process. An isothermal process has the added benefit of reducing the sensible load on the downstream evaporative components. Some researchers have paid attention to internally cooled, liquid desiccant-based devices to achieve an isothermal dehumidification process. For example, a one-dimensional numerical model was adopted to analyze the performance of the internally cooled liquid desiccant–air contact units, with the results proving that the dehumidification capacity could improve due to the isothermal process.
Model and experimental studies of an internally cooled/heated dehumidifier/ regenerator of liquid desiccant systems used an internally cooled dehumidifier and internally-heated regenerator. Those studies showed that avoiding the adiabatic dehumidification process could offer higher regeneration efficiency for the dehumidifier. Other researchers have focused on the two-stage rotary desiccant cooling system to achieve an isothermal dehumidification process. The present study explains the role of desiccant cooling as a green air conditioning technology to provide the fresh air for maintaining required indoor thermal comfort for human hygiene.
Effect of humidity variations on human health
Over the last few decades, the quality of air in indoor environments such as houses, apartments, and offices has been extensively investigated both for residential as well as various industrial applications. Field studies have frequently found undesirably high levels of known respiratory irritants such as nitrogen and sulphur dioxides, hydrocarbons and other particulates which come into existence due to the pollution in air due to automobiles and known or suspected carcinogens such as asbestos, radon, some particulates and formaldehyde etc., due to pollution created by industrial waste. In many cases, high indoor levels of contaminants have been traced to indoor building materials, furnishings, appliances, and human activities.
Indoor contaminant levels can also be exacerbated in tightly sealed energy conserving buildings with minor fresh air ventilation rates. Either lowering the sources of pollutants or increasing ventilation rates, or both, can be used to reduce or eliminate the levels of these contaminants. Water vapour, usually measured as relative humidity or the percentage of water vapour held by the air compared to the saturation level, is not usually considered to be an indoor contaminant or a cause of health problems. In fact, some level of humidity is necessary for comfort. On the other hand, the relative humidity of indoor environments (over the range of normal indoor temperatures of 20 to 26°C, has both direct and indirect effects on health and comfort. The direct effects are the result of the effect of relative humidity on physiological processes, whereas the indirect effects result from the impact of humidity on pathogenic organisms or chemicals.
This review is primarily concerned with the indirect health effects of relative humidity, which are more complex than the direct health effects and of greater public health significance. However, it is worthwhile to briefly discuss some of the direct health effects, as these effects often lead to solutions such as dehumidification carried out by the desiccant cooling, which may in turn indirectly affect health. Both extremely low and high relative humidity may cause some physical discomfort, as the relative humidity of the air directly affects temperature perception. Very low level about below 20% RH may also cause eye irritation and moderate to high levels of humidity reduce the severity of asthma. Several reports, apparently based on the experience of physicians with patients who complained of dryness of the nose and throat during extremely lower relative humidity, have also argued that indoor relative humidity should be kept above 40 to 50% in order to prevent drying of the mucous membranes and to maintain adequate nasal mucus transport and ciliary activity in human body. These known or suspected adverse effects of low relative humidity have led to the wide spread use of humidifiers in areas where cold winters lead to low indoor humidity.
Relative humidity also has an important adverse direct effect on health when high humidity is combined with high temperatures. This combination reduces the rate of evaporative cooling of the body and can cause considerable discomfort due to excessive perspiration and so lead to heat stroke, exhaustion, and possibly death. Case reports and epidemiological studies reveal that relative humidity and humidification equipment can indirectly affect the incidence of allergies and infections.
Both relative humidity and humidification equipment have influence on the population growth and survival of infectious or allergenic organisms such as fungi, protozoans, mites, bacteria and viruses, as well as the probability of effective contact (exposure that results in disease or adverse symptoms) with indoor spreading of these organisms in hospitals and houses. These indirect effects may partially account for the suspected relationship between respiratory infections and nose or throat irritation and relative humidity.
In addition, relative humidity affects the concentration of noxious chemicals in the air by altering the rate of off gassing from building materials and by the reaction of water vapour with chemicals in the air. A review of the available data on the indirect health effects of relative humidity shows that these effects do not uniformly increase or decrease in frequency or severity with a change in relative humidity. Instead, for a given relative humidity, some adverse health effects can be at a maximum while others are at a minimum. The relative humidity range for minimizing as many adverse health effects as possible appears to lie between 35 and 50%. The evidence to support this optimum relative humidity range is presented below Figure 1.
Effect of uncontrolled humidity in healthcare
Humidity in healthcare facilities can cause microbiological growth, discomfort for surgeons, the deterioration of critical medical equipment, and an unsafe environment for patients’ recovery. Controlling humidity in hospitals is extremely important, but the fear of mold and bacteria is not the only factor for the requirement of dehumidification. Surgeons are demanding cooler, drier operating room conditions. Hospital administration has prioritized indoor environmental conditions, not only for the safety of the patients, but also to improve the efficiency and quality of work life for the surgeons.
Hospital applications require cooler temperatures and lower dew points. Desiccant technology is an optimal method to achieve these operating room conditions and helps reduce the cost of equipment investment. Hospital engineers are challenged with the daunting task of employing HVAC systems that provide the proper amount of fresh outside air, HEPA filtration, air changes, and temperature and humidity control. The proper selection and sizing of dehumidification equipment is critical for the HVAC system efficiency and basic hospital operation.
When indoor air is too moist or has RH levels generally above 60% — occupants can face a different range of health hazards including:
- Mold Growth: Mold requires water, food (organic matter), and oxygen to grow. High humidity levels in a facility can supply the water mold needs to grow. The EPA affirms that mold can cause respiratory illness, allergic reactions, and other types of illness and irritations. One study found that 21% of 21.8 million cases of asthma annually can be attributed to residential dampness and mold.
- Dust Mites: Absorbing water through the air, dust mites thrive in high humidity environments. They are known to be one of the major indoor triggers for allergies and asthma.
- Fungal and Bacteria Growth: Most fungal species cannot grow in RH below 60%. Additionally, pathogenic microorganisms can adhere more effectively to moist materials and textiles.
A number of actions and events can cause fluctuations in humidity – from rainy outside weather to cooking to showering. However, one of the most common contributing factors to clients’ humidity control issues is a space’s air temperature and how it relates to the HVAC system air temperature – specifically a misconception about the relationship between temperature and relative humidity. Generally, if air temperature decreases, absolute humidity also decreases – because air’s ability to hold moisture increases as the air gets warmer.
First, a differentiation needs to be made between space air temperature and the HVAC system air temperature. The average building occupant typically has the ability to modify the space air temperature (using the thermostat setting). However, the HVAC system air temperature is a product of the overall HVAC system design and directly limits the range of conditions (temperature and humidity) of the associated spaces. The air provided by the HVAC system (when in cooling mode) can be simplified to a dew point temperature. Dew point is the temperature to which air must be cooled to become saturated with water vapour – correlating to 100% RH at the given temperature.
Relative humidity is relative to the temperature, conditions, and limitations of a facility’s atmosphere and mechanical/cooling systems. This situation often occurs in healthcare facilities as mechanical systems may be designed to operate within temperature standards that are higher than what end-users, such as surgeons and doctors, may prefer. If a hospital or medical office building’s cooling system was designed with a warmer dew point that correlates to 68-72 degrees space temperature, but staff are running it daily at 62 degrees, this will lead to high humidity in the cooling season. Humidity issues like this are often thought to only affect facilities located in hot, muggy climates. However, with the above chart in mind, humidity control can be an issue in any climate and any facility whose mechanical systems are being operated outside of their design parameters.
Use of desiccant cooling in effective humidity control in healthcare
As shown in Figure 2, a type of arrangement is made in which solid desiccants such as silica-gel, molecular sieve etc., are packed to form a sort of adsorbent beds exposed to the incoming airstream that takes up its moisture. The beds are periodically moved in the direction of the regeneration air stream and then returned to the process air stream as shown in Figure 2. Liquid desiccants are often sprayed into air streams or wetted on to contact surfaces to absorb water vapour from the incoming air, which latterly like the solid desiccants regenerated in a regenerator where water vapours previously absorbed are evaporated out from it by heating. To eliminate the overcooling and the reheat, the desiccants can also be coupled with the traditional air conditioning system, thus reducing the equipment size and their costs. Their most frequent use remains, however, their employ with the evaporative cooling.
Indeed, the evaporative cooling is the oldest technique of cooling. The desiccant cooling is found more efficient than the conveniently operated conventional air conditioning – subsequent technology has suppressed this old technique. But due to the energy costs and the concerns related to environmental harms engendered by the refrigerants used in this system, the researchers began looking back at the old cooling technique and trying to solve their main drawbacks. These techniques’ main drawback is their operational inefficiency in very humid climate, and even for the tropical and dry climate, their seasonal operating inefficiency (even in tropical climates, they become inefficient in rainy seasons). Desiccant cooling emerged as a solution to this problem. By dehumidifying the incoming air forcing it through the desiccants, the evaporative cooler can achieve greater efficiency rather on the dry air stream.
Air distribution in healthcare buildings
Figure 3 shows the introduction of low velocity air near the ceiling at the entrance of the room, flowing past the patient, and exhausted or returned close to the floor at the head of the patient bed. An airflow pattern is thus established which helps to move microorganisms from the point of patient’s expulsion to the exhaust / return air terminal to prevent health care workers or visitors from inhaling the bacteria.
Conclusion
The adverse health effects of higher relative humidity concentration may be growing in importance as a result of the continuing construction of energy efficient sealed buildings cooling and efficient moisture control with low fresh air ventilation rates. The high fresh air ventilation rates found in older leaky buildings may dilute the concentration of pathogens, allergens and noxious chemicals in the indoor air and thus offset some of the health problems associated with relative humidity In contrast, energy-conserving buildings require the careful maintenance of good indoor air quality through maintaining, among other factors, optimum relative humidity levels in order to minimize potential health problems.
As conclusion of this overview, the need for a lower limit for long term indoor air humidity range can be deduced, but further research is needed to gain deeper knowledge about the effects of low indoor air humidity. This knowledge will help to save a lot of technical expenditure as well as energy for indoor air conditioning. Use of desiccant cooling in maintaining and controlling relative humidity as environmental thermal control that can be driven by low grade heat, is considered as a promising method for assuring clean and economical air conditioning. The main objective for using desiccant materials is to remove latent heat in predetermined cycles. The development in desiccant technology is in progress in terms of its desiccant materials and it is attaining stability in the market. It appears to be reliable, safe, and environmentally friendly according to the needs of our society to maintain good health.
Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is a recognized Ph.D. Supervisor at Gujarat Technological University (GTU). He has published more than 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books in the area of thermal engineering. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of Gujarat, India). His area of research is Desiccant cooling, ANN, TRNSYS, and Exergy.
