Humidity control in critical areas of hospitals

*The values are taken from the normative annex to NP 015-2022.

A hospital has a range of humidity requirements, and the design is based on the function of each room.

For operating theatres, NP 015-2022 specifies a range of 30–60 per cent RH. Note 3 of the table refers to Order No. 914/2006 of the Minister of Public Health and specifies that the ranges may differ in other standards, depending on the filtration stages and filter types.

For wards designated for patients with severe burns, the standard specifies a separate range of 60–95 per cent RH. High humidity helps to limit water and heat loss through skin that has lost its barrier function. Microclimate conditions are also detailed in Order of the Minister of Health No. 476/2017, Annex 1.

The system must be designed so that each zone meets the specified parameters. Depending on the HVAC configuration, separate controls and different humidification solutions may be required.

The difference extends beyond the values in the table. The burns unit falls within Class I rooms, and the standard requires Class I rooms to have systems with 100 per cent fresh air, without recirculation. In Class II operating theatres, partial recirculation of up to 50 per cent may be used, with UV decontamination on the circuit as specified by the regulations.

Two areas within the same building may thus differ both in terms of indoor parameters and the proportion of outside air. Both factors directly influence the humidification load.

What do studies show about low humidity levels?

Below 40 per cent relative humidity, the mucous membranes in the nose and throat can dry out, and the respiratory tract’s natural defence mechanisms may function less effectively. The link between air humidity and the body’s response to respiratory infections was analysed by Moriyama et al., 2020.

In dry air, exhaled droplets evaporate more quickly and can reach sizes that keep them suspended for a longer period. However, the dynamics depend on the type of particles, the virus and the conditions in the room.

In the experiment conducted by Noti et al., the influenza virus lost its ability to infect more quickly when the relative humidity exceeded 40 per cent.

A Mayo Clinic study led by Jennifer Reiman monitored four nursery classrooms over the course of a winter, two of which were humidified. In the humidified classrooms, approximately one-third less influenza virus was detected in the air, and the virus found remained infectious in only 17 per cent of samples, compared with 48 per cent in the non-humidified classrooms.

The studies cited recommend avoiding prolonged periods of very dry air, but do not in themselves establish the design value for an operating theatre nor do they alter the 30–60 per cent range specified by the regulations. The target value for operational use is established during the design phase, depending on medical requirements, the installation and operating conditions.

During cold spells, outdoor air drawn into the building and heated can reach a much lower relative humidity. Without active humidification, indoor humidity may fall below the design values for prolonged periods.

Where does the sizing of a humidification system begin?

Selecting a humidifier begins with determining the amount of water that needs to be added to the air to maintain the required humidity. This amount, usually expressed in kilograms of water per hour, represents the

The calculation is based on:

• the air flow rate into the room;
• the proportion of outside air;
• the temperature and humidity of the outside air under the design conditions;
• the required indoor temperature and relative humidity (RH);
• the system’s operating schedule.

The room’s floor area provides too little information for this calculation. Two rooms with the same floor area may have different air flow rates and proportions of outside air, leading to very different humidification capacities.

A correct calculation prevents two situations: selecting equipment that produces too little steam to achieve the design value, or selecting a capacity greater than is actually required.

Humidity is designed in conjunction with pressure and air flow rates

NP 015-2022 addresses temperature, humidity, air flow, filtration and pressure conditions collectively. All these factors influence how air circulates through the room and the conditions that can be maintained during medical procedures.

For the operating theatre, the standard requires a pressure differential of at least 10 Pa compared with adjacent rooms.

The operating theatre operates under positive pressure: the pressure inside is higher than in the adjoining areas, and air flows from the theatre outwards. This direction of air flow limits the ingress of contaminants from areas with lower aseptic requirements.

The pressure differential must be maintained at all times and monitored using measuring devices. The system can also emit an audible alarm when the pressure falls outside the set limits. The values can be monitored locally or via the building management system (BMS).

Depending on the asepticity classes being separated, the standard specifies a maximum pressure differential of between 10 and 20 Pa.

In the centralised system described in the standard, the exhaust air transfers some of its heat to the fresh air via a heat exchanger using an intermediate fluid. This heat recovery alters the air temperature before it reaches the humidifier. The calculation of the humidification load must use the air parameters resulting from this stage.

How is the 30–60 per cent RH range maintained in an operating theatre?

In a centralised system, the air passes through several stages before reaching the operating theatre:

1. filtration, stage I;
2. heat recovery;
3. preheating;
4. cooling and dehumidification;
5. heating;
6. flow control;
7. filtration, stage II;
8. steam humidification;
9. sound attenuation.

The third stage of filtration, using a HEPA or ULPA filter, is fitted as close as possible to the air inlets. These filters capture very fine particles before the air reaches the operating theatre.

The steam is introduced after the second stage of filtration. From that point, it requires a certain length of pipework to mix and be fully absorbed into the air. This is known as the absorption distance.

The first elbow, filter, sensor or other obstruction must be positioned beyond this distance. Steam that comes into contact with a component before it has been fully absorbed may form condensation. Near the final filter, condensation can dampen the filter material and impair its performance.

The hall’s schedule is also important. The system must be switched on early enough for the temperature, humidity and pressure to reach the designed values before the first event.

What must be provided for in the system

• humidification capacity calculated for the relevant winter conditions;
• the space required for the equipment and the steam distribution system;
• the absorption distance in the pipework;
• the position of filters, sensors, bends and other components;
• sensors for measuring humidity;
• sufficiently rapid control in response to load variations;
• condensate drainage, where required by the technology;
• connection to the air handling unit;
• monitoring and alarms via the building management system;
• access for inspection, cleaning and servicing.

How do the requirements differ between the operating theatre, the intensive care unit and the burns unit?

The same calculation method applies to all areas, but the values entered into the calculation and the operating mode differ.

Intensive Care Unit

For anaesthesia and intensive care, NP 015-2022 specifies a temperature of 24–26°C and a relative humidity of 35–60 per cent.

The systems serving the ICU operate continuously. Equipment capacity, automation and maintenance must be designed to maintain these parameters throughout the day.

The design may include:

• available capacity for continuous operation;
• continuous monitoring of temperature and humidity;
• alarms for deviations from the set ranges;
• redundancy, depending on project requirements;
• access for maintenance whilst the area remains operational.

Redundancy means the existence of reserve capacity. If one piece of equipment requires servicing, the other can take over all or part of the required load.

Reserve for burn patients

For the ward designated for patients with severe burns, the standard specifies a temperature of 26–32°C and a relative humidity of 60–95 per cent. The area is classified as Class I and operates exclusively with 100 per cent fresh air.

In winter, the cold outside air contains very little water. After being heated to 26–32°C, its relative humidity drops even further. The system must add a large amount of vapour to achieve the required value in the ward.

For this reason, the burn unit may have the highest humidification load of any area in the hospital. Capacity is verified using the design outdoor temperature and humidity for winter, the total fresh air flow rate, and the indoor value specified in the design.

The equipment is only part of the solution. The air handling unit, pipework, steam distribution, sensors and automation must all work together to maintain the design value.

Any change to the air flow rate, the proportion of outdoor air, or the indoor temperature or humidity requires the calculation to be redone.

5 decisions to be made during the design phase

System integration becomes simpler when capacity, space, utilities, automation and maintenance are established from the outset.

1. Required capacity

The calculation uses:

• the room’s function;
• the design temperature and relative humidity;
• supply air flow rate;
• the proportion of outdoor air;
• the design outdoor conditions;
• operating schedule.

These figures indicate how many kilograms of water per hour the system must produce. Calculating based on room area may result in an inappropriate capacity, as the load depends primarily on the quantity and condition of the supply air.

2. Space for equipment and steam distribution

The design must allow space for:

• a humidifier;
• the steam distribution ramp or pipes;
• the absorption distance;
• water and power connections;
• drainage;
• access for servicing.

If the distance between the steam distribution point and the first elbow, sensor or filter is too short, this encourages the formation of condensation. The position of the humidifier and the pipework layout must be analysed together.

In an existing installation, the assessment begins with the available space and the components already fitted. These determine the technical options that can be incorporated.

3. Water, drainage and power supply

Each technology has its own supply requirements.

The steam generator requires:

• water of the quality and flow rate specified by the manufacturer;
• drainage sized for the flow rate and temperature of the discharged water;
• pipes and materials suitable for hot water;
• electricity, gas or steam from the building’s network, depending on the chosen solution.

The required power may influence the sizing of the electrical distribution board. For gas-fired solutions, the design must include the supply and extraction of combustion gases. For equipment supplied with steam from the building’s network, the pressure, quality and availability of the steam must be checked.

4. Sensors and integration with the building management system

Sensors measure humidity, and the control system adjusts steam production to variations in flow rate and room conditions.

The following are determined during the design stage:

• the position of the sensors;
• the parameters to be monitored;
• alarm limits;
• the signals transmitted;
• the points displayed in the building management system;
• the communication protocol.

Certain Condair units allow communication via Modbus or BACnet. These protocols transmit measured values, equipment status, alarms and information required for operation to the building management system.

5. Maintenance and operational continuity

Access for servicing must be adapted to the location of the equipment and the operating hours of the area.

The design must allow for:

• inspection and cleaning of the equipment;
• access to components that are replaced periodically;
• emptying and sanitising;
• servicing;
• the area to remain operational during maintenance, where the design requires continuity;
• the inclusion of a standby capacity, where risk and function so require.

Equipment installed above a suspended ceiling or in a hard-to-reach area prolongs each service call and may require the system to be shut down. The service space must be checked before the final position is determined.

These five categories of information form the basis for sizing. The project phase completes the analysis: in a new build there is greater freedom of positioning, whilst a retrofit is constrained by the existing space, routes and utilities.

Do you already have the HVAC diagram or project data? TEXCO can check the humidification requirements and determine the appropriate Condair configuration for the planned medical area.