HUMIDITY CONTROL
bug’) survive longer at an RH of 20-30%, while a mid-range RH between 40% and 70% will minimise their survival rate. Other tests indicate that the infectivity of the influenza virus is increased by both low and very high RH, with the minimum infectivity at 50% RH. RH has been shown to have a similar effect
on airborne bacteria, with a high mortality rate of airborne pneumococci, streptococci and staphylococci at intermediate RH levels.
RH and surface-borne infections Surface-borne infections must also be addressed. It is well-known that when RH is very low, static electricity is an issue and raising the RH above 35% allows surfaces to become covered in a thin film of moisture that dissipates the static charge. This also facilitates more effective cleaning. It has also been discovered that survival of Methicillin Resistant Staphylococcus Aureus (MRSA) was significantly reduced when contaminated surfaces were stored in 45-55% RH, compared to storing them at 16% RH. Moreover, it is also well-known that some
types of bacteria will form protective spores at an RH of less than 40%, making them more resistant to the actions of surface disinfectant.
RH and infection resistance
Persistent low RH (below 40%) also has a direct impact on the human body. Breathing dry air causes moist tissues in the nose, throat and lungs to dry out, leading to damage which would render the body susceptible to infections. Therefore, raising the RH to a mid-range not only reduces the survival rates of viruses and bacteria, it also supports the body’s ability to resist infection. Dry air will increase the rate of evaporation from tissues during an operation, thereby exacerbating problems of dehydration.
Taking control
As noted earlier, an RH range of 40-70% is considered acceptable for most workplaces and
will also be suitable for most healthcare facilities, though it’s important to check specific regulations relating to particular areas in hospitals and other facilities.
For example, the Health Technical Memoranda (HTM) Guidelines 7.48 and 7.50 define the acceptable range of humidity as between 35% and 60% saturation. The Scottish Health Facilities Note 30, Infection control in the built environment: design and planning, notes that control and physical monitoring of humidity can help ensure that environmental conditions do not contribute to the spread of infection.
Problems relating to RH in the UK tend to be because of low humidity. During the winter months buildings are usually heated to around 20°C and as the temperature rises the RH falls dramatically. For example, outdoor air at -5°C and 100% RH has a moisture content of 0.0025kg moisture per kg of dry air. When raised to 21°C dry bulb, with no humidification control, the resultant RH is 18%, which is very dry. Where comfort air conditioning is used in summer to reduce temperature, the cold surfaces in the air conditioning system also remove moisture from the air, again causing low RH.
Thus, humidity control usually requires the
introduction of moisture using a humidifier to raise the RH. To that end, HTM 03-01 Part A Guidelines 3.55, 3.56 3.59 and 3.60 prefer clean, dry (central plant) steam for humidification. However, if that is not available a steam generator should be provided locally. It also states in Guidelines 4.91 to 4.115 and 4.91 to 4.114 that only steam injection systems are permissible for humidification, and that water curtain, spray or mist humidifiers should not be used. At this stage, it’s important to stress that specifiers and installers need to acquaint themselves with the specific regulations that apply to the project with which they are involved. It also makes sense to engage with specialists in the field of humidity control as steam for humidification can be generated in a number of different ways, using either electricity or gas to provide heat.
Arriving at the best solution with the right balance of performance and cost of ownership means taking a look at a number of important criteria.
Hum Sols
www.acr-news.com September 2017 45
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