is highlighting a recently published US study which it says backs over 60 years’ evidence that carefully controlling humidity in operating theatres and wards could significantly reduce transmission and survival of both the influenza virus, and potentially also other infection and virus-causing organisms. Jonathan Baillie reports.
Discussing with me the latest research on the impact of humidity on the influenza virus was Vapac’s national sales manager Dave Mortimer. Right from the start of our discussion, which took place in an office next to the large factory in Edenbridge, Kent where the company assembles its range of electrode boiler and resistive heating humidifiers for healthcare, educational, industrial, retail, and office applications in both the UK and overseas, he made clear his own strong belief in the scientific arguments for greater use of hospital humidification in combating the spread of both influenza, and possibly (depending on the results of several ongoing studies), other bacteria and viruses. Indeed evidence of the impact of, for instance, maintaining relative humidity at between 40 and 60% on the viability and survival of several strains of “flu” had, he told me, been welldocumented since the 1940s. Two key recent developments had, however, re-focused debate on the issue – guidance in the latest Healthcare Technical Memorandum covering heating and ventilation of healthcare premises (HTM 03-01, published in November 2008) that appears to indicate a change in Department of Health thinking on the need to install humidifiers in some operating theatres, and publication of a US study that focuses on the potential effect on influenza survival and transmission of both relative (RH) and, more particularly, absolute humidity (AH) levels. The US paper, (Absolute humidity modulates influenza survival, transmission, and seasonality, Sharman J., Kohn M.), was authored by Professor Jeffrey Shaman, assistant professor at the College of Oceanic and Atmospheric Studies at Oregon State University in the US, and Melvin Kohn of the Public Health Division at the Oregon Department of Health Services. One of its key conclusions is that, “while for certain organisms relative humidity may indeed affect biological response” (endorsing the belief that maintaining RH between set parameters in hospitals may well reduce influenza spread), the absolute humidity (AH), i.e. the actual water vapour content of the air irrespective of temperature, “can be of greater biological significance for many organisms”. The study goes on to suggest that absolute humidity is, in fact, the “defining factor”, and can “greatly reduce both the viability of the flu virus, and the transmission of influenza”.
Further research needed
The paper emphasises, however, that further research is needed to identify, in particular, how the virus is affected on a molecular scale by particular humidity levels, and, having been undertaken on guinea pigs, that similar research needs to be undertaken on humans into the degree to which AH constrains influenza virus survival (IVS) and influenza virus transmission (IVT) rates. Dave Mortimer told me he was convinced, nevertheless, that that the study’s findings, coupled with over half a century’s research evidence on the same topic, but which has generally tended to focus more on the impact of relative than absolute humidity (since it is RH, not AH, which an estates or clinical team will adjust in ward or theatre via thermostats and humidistats), points to a significant opportunity for UK healthcare facilities to substantially reduce influenza transmission rates at “a fairly negligible cost” via wider use of humidification equipment. He said: “Study evidence that colleagues and I have regularly come across dating back many years suggests that if relative humidity in, say, a hospital ward, falls below about 40%, the waterbased influenza droplets expelled by patients coughing and sneezing shrink rapidly due to evaporation. Consequently they remain airborne, and are thus easily transmitted from patient to patient, and indeed from patients to staff and visitors. In contrast, if you can maintain RH at, say, between 40 and 60%, the droplets remain larger, and tend to settle on surfaces where, particularly with today’s arsenal of powerful cleaning products, they can easily be wiped away, “Interestingly,” he continued, “with some of the major recent major flu epidemics, especially in the Far East, people have rushed out to buy masks. However they may well be wasting their time, because so small do the influenza droplets become when RH falls between a certain level that the microbes will easily pass through the mask, rendering them ineffective. “In contrast,” he added, “if a hospital can keep RH at between 40% and 60% in theatres and wards, and, importantly, it appears from Professor Shaman’s latest research, maintain absolute humidity at a corresponding level, then aerosolised droplet distribution, and the survival rate of the flu virus, may be significantly reduced.”
Easier to clean surfaces
Dave Mortimer also pointed out that many studies, and indeed anecdotal evidence from hospital cleaning teams, confirmed that hospital surfaces in rooms where relative humidity is kept at the suggested levels are much easier to clean. He explained: “Although the layer of moisture in such environments is not visible, it is nevertheless still there, and greatly facilitates cleaning with the many hi-tech cleaning materials and agents hospitals now have at their disposal.” While he could appreciate why a recent UK Government public health campaign had advised cold and flu sufferers to “catch it, bin it, kill it” in terms of dealing with phlegm and mucus expelled from the nose and throat, he believes one of the shortcomings of the approach is that, should relative humidity levels fall below around 40%, as they will in many UK hospital wards when the heating is turned up to cope with winter temperatures, minute virus droplets will still remain in the atmosphere. He said: “The latest Government advice also depends on everybody, and even those unaware they have influenza, following the rules, especially in enclosed environments such as hospitals, offices, theatres, and conference halls.” One of the questions that appears to have vexed public health administrators and policymakers in the UK for some time is the seasonality of the flu virus, with a tendency for a marked increase in cases during cold winter months. In common with many informed observers, Dave Mortimer has always believed the explanation may be quite simple: that, as outside temperatures drop, and hospitals turn up their heating, unless humidifiers are installed relative humidity levels inside the building will fall, the air will become drier, and prevalence of airborne influenza droplets will proliferate, a phenomenon likely to occur not just in hospitals, he says, but equally in many other enclosed public spaces.
A ‘pronounced seasonal circle’
Interestingly, however, Professor Shaman’s latest study contends that “such assertions do not square with the fact that outdoor RH peaks in winter”, adding that: “In contrast, both indoor and outdoor AH have a pronounced seasonal cycle marked by a prominent winter-time low”. In further reinforcing the apparent importance of levels of absolute humidity on influenza virus survival and transmission, the US study’s authors add: “The evidence presented here indicates that both IVT (influenza virus transmission) and IVS (influenza virus survival) are much more tightly constrained by AH than RH, and that seasonal changes in AH match seasonal patterns of influenza incidence better than seasonal changes in RH.” The paper continues: “Recent theoretical work has shown that slight seasonal changes in transmission efficiency can produce the strong seasonal cycle in influenza incidence through dynamic resonance. The findings presented here indicate that the seasonal cycle of AH may be the factor producing this resonance. Specifically, low wintertime AH levels, both indoor and outdoor, increase influenza IVS and thus increase IVT efficiency. These findings also suggest that humidification of indoor air, particularly in places where transmission could be to those at high risk of complications, such as nursing homes and emergency rooms, may help decrease the spread and the toll of influenza during influenza season”. In Professor Shaman’s study he and coauthor Melvin Kohn confirm that previous studies using RH “also supported similar air humidification practices”. However the authors say these “assertions” were “perhaps undermined by the marginal association of RH with IVS and IVT”. The authors also acknowledge that the precise mechanisms via which AH affects influenza virus survival are not understood; however their findings indicate that IVS “responds to the amount of water vapour in the surrounding air (i.e. AH), and not how close that air is to saturation (i.e. RH)”. They say: “It is also not yet clear why the stability of an influenza virus encased within a droplet nucleus would be sensitive to atmospheric AH conditions; thus an exact determination of how IVS is affected by AH, and whether there exists any change in the size distribution of expelled particles, including coughed or sneezed droplets, under different environmental conditions, is needed.”
‘Baffled’ by inaction
Dave Mortimer says he finds it “baffling” that, while many studies have suggested that low humidity causes higher rates of airborne influenza transmission, to date, at least, the implications seem to have been largely ignored by those, such as hospital boards, with the power to influence change. One of the many study examples Vapac alludes to is one published in 1960, Virus survival as a seasonal factor in influenza and poliomyelitis ( Hemmes J.H., Winkler K.C., Cool S.M.). So far, Vapac emphasises, much of the scientific study and research evidence has highlighted the impact of carefully controlled humidity levels on various strains of influenza. However Dave Mortimer explained that there were several other studies ongoing that were seeking to reinforce strong anecdotal evidence that “keeping humidification levels high, but not too high” could also help reduce transmission of other bacteria and viruses, including one of today’s most familiar “superbugs”, MRSA. Returning to the existing flu virus issue he added: “At this time of year many UK hospitals will be turning up the heating. In most operating theatres, certainly in larger and newer acute hospitals, air handling plant will already incorporate humidifers, so relative humidity should, in theory, be maintained at the recommended level of around 50%; thus the influenza droplet problem should not be widespread. However, in many wards, where heating may be being pumped out into the atmosphere, and temperatures may reach well over 22 degs C at times, the lack of humidification equipment will see relative humidity levels fall to 25% or below. In such environments the droplets expelled into the air will simply evaporate and linger in the atmosphere, causing more and more people to contract the flu virus.”
Time lost to illness
Vapac concedes that “it is far too simplistic, and there is no evidence to suggest, that thousands of pounds could be saved on vaccinations by the mere inclusion of humidity in a building”. However it points out that “it takes time to develop vaccines”, and that the costs to business and to staff shortages within essential services, particularly in the NHS, via lost man hours resulting from colds and flu, must be taken into consideration, “along with the reduced chance of catching swine flu.” “What is extraordinary,” Dave Mortimer said, “is that if you talk to many clinicians, engineers, and some of the better-informed estates and facilities managers, most are aware of the scientific evidence that controlling humidity helps to kill bugs. Despite such awareness, however, there seems to have been little concrete action in the UK, in contrast to the US and Canada where, for instance, humidifier specification is widespread when building new hospitals, and the practice’s benefits are well-known.” Some apparently powerful arguments, but with NHS budgets being squeezed, what of the costs of installing new humidification equipment, or retrofitting it in existing plant rooms? Dave Mortimer said: “Coupled to the obvious clinical and administrative cost savings to the NHS of reducing flu case numbers, influenza can seriously threaten some sufferers’ health, and particularly that of older, frailer people, and the immuno-compromised. Added to this, the additional cost of installing a humidifier appropriate to a particular operating theatre or ward is often relatively low.” When pressed on the specifics, he added: “Although the actual cost would depend on several factors, such as air flow, the size of the room, and the application, a single unit could sell for as little as £2,000. A humidifier will clearly consume electricity, and, in the case of our electrode-based units, require an occasional change of plastic cylinders, but otherwise there are few ongoing costs to worry about.” In addition to citing Professor Shaman’s research as a reason for hospitals to “at the very least give more serious consideration” to installing humidification equipment, Dave Mortimer said there was another important reason why Vapac had become so animated about the issue. He explained: “While the previous Department of Health Health Technical Memorandum governing ventilation of healthcare premises, HTM 2025, indicated that installing a humidifier in operating theatres should be considered the default approach, the newer HTM 03-01 seems to imply that such a step is not necessary in “general” theatres. (see panel, “What the HTM says” on page 56 for fuller details on the wording in HTM 03-01). However in my, and I know Vapac’s view, any operating theatre where invasive surgery will take place should be humidified at the generally accepted level of around 50% RH.”
Changes in medical gas policy
The Vapac humidification specialist believes the change in the newer HTM’s recommendations is explained by the fact that, in the past, with considerably greater use of flammable medical gases, Department of Health policymakers were principally concerned about the risk of explosion or fire in very dry atmospheres, and were also wary of potential static problems, both as regards machinery, and dust and other airborne debris in the rarefied, “cleanroom environment” of a typical operating theatre. However with the use of such medical gases now less prevalent, and static less of an issue than in the past, the writers of HTM 03-01 have apparently taken the view that they no longer need to recommend humidifier installation in “general operating theatres” as a matter of course. Dave Mortimer says this is a significant worry for Vapac since, especially against today’s tough financial backdrop, hospital Trusts and other healthcare providers will often be looking to reduce costs when building and equipping new hospitals, and any suggestion that humidification is no longer essential may thus be seized upon as a “get-out”. He added: “Up until recently, especially given the advice in the former HTM, most UK hospital operating theatres will, as a matter of course, have been fitted with humidification equipment, enabling, say the surgeon, or another operating team member, to use a sophisticated control panel to adjust the relative humidity to the correct level, in the same way as they regulate the temperature.
Asking surgeons
“If you ask most surgeons how important maintaining RH in the theatre at around the recommended 50% level is they will tell you it is vital. When you think that the human body is largely made up of water, and that when a surgeon opens a patient up there is the opportunity for the human tissue exposed to dry out, you can appreciate why correct humidity levels are essential, not just from an infection control, but equally from a clinical, perspective.” In the ward environment, meanwhile, Vapac says that, with the Department of Health, CIBSE, and environmental organisations encouraging architects and hospital constructors to ensure that new healthcare buildings are well sealed and insulated to minimise energy use and carbon footprint, the air inside many, especially if air conditioned but not humidified, will simply get drier. One recent acquaintance’s experience confirmed this; with the lady concerned recounting how, on visiting her mother recently in a fairly newly built NHS hospital, she discovered that nursing staff had placed small plastic bottles of water on the radiators once the heating was turned up, as it was “the only way to make the environment comfortable”. Dave Mortimer said: “So far we have mainly focussed on the potential infection transmission reduction potential of improved humidity control, but you must, of course, also consider the patient comfort factor. Patients already unwell are hardly likely to benefit from lying in a ward where the air is extremely dry due to lack of humidification. In the summer, meanwhile, the air conditioning may be operating at full tilt, but without a humidifier built into the air handling plant much of the moisture will be sucked out, creating an uncomfortable atmosphere.” The problems of lack of sufficient humidity, Dave Mortimer also pointed out, could often be less immediately apparent to patients and staff than a significant temperature change, which most people would notice extremely quickly.
Debate re-opened
Vapac says it is pleased the paper from Dr Shaman and his colleague in the US has re-opened the debate on the benefits of humidification, and argues that, if more heed is taken both of the latest study evidence, and that of the past 40-50 years, there could be substantial public health benefits. However there are some caveats. One that Dave Mortimer is especially keen to emphasise is that Dr Shaman (and indeed other scientists’) research indicates that some pathogens, such as poliomyelitis, actually thrive in high humidity, so the use of a humidifier can have some “unintended consequences”. Fortunately,” he told me, “the evidence is quite clear: that if relative humidity is maintained at between 40% and 60%, airborne transmission rates of many bacteria and viruses will be significantly reduced. The issue, therefore, is to get the levels just right – something which should not pose too much of challenge provided the user consults a humidifier manufacturer who can provide specialist advice and an appropriate solution for the application.” One important consideration here was to ensure that customers fully understood the difference between relative and absolute humidity. Vapac says potential confusion has at times seen it encounter users specifying humidification equipment “after getting their sums wrong” and then experiencing problems. For instance should customers not take sufficient account of the need to vary (steam) absorption length to cater for potential changes in outside temperature and of airflow post-installation, the filters in the humidification plant may “wet out”, and then not function optimally. In general terms, Vapac explains, when specifiers, suppliers, and users of humidification and other air handling plant talk about humidity, it has become the “accepted norm” to use relative humidity, or RH, as the benchmark. Dave Mortimer says: “The majority of our control strategies are based on RH, but it is important for estates and facilities managers, and other specifiers, to understand both relative and absolute humidity and what is meant by each. “For those not clear, relative humidity is the ratio of the amount of water vapour in the air to the humidity level of the air were it saturated, expressed as a percentage. Absolute humidity, on the other hand, is the amount of water vapour per volume or weight of air.” Returning to the potential opportunity for greater UK use of humidification in wards and theatres, and, in the process, the possibility of significantly reducing hospital transmission of influenza and other harmful bacteria and viruses, Dave Mortimer said: “Why more notice has not been taken, at least in the UK, of the clear scientific evidence that exists on the topic, is a mystery to me, and indeed to many of my engineering colleagues. Although Professor Shaman’s latest study makes clear that more research must be done, there is little question in my mind that the UK health sector could reduce rates of transmission of influenza, and potentially, in future, also of other airborne pathogens, using a technology that, particularly if designed into a hospital scheme from the outset, is both relatively simple to install, and extremely cost-effective to operate.”
Retrofitting’s implications
Vapac says that retrofitting humidifiers is “certainly an option” for hospitals and other healthcare facilities, but, for instance, where plant room space is limited, adding the equipment to existing plant will often be harder than incorporating it from the outset. Dave Mortimer said: “We will now be watching developments in this area with increasing interest, but are convinced that more accurate control of humidity in many hospitals, and indeed installation of humidifiers in theatres, but more predominantly, wards where they are currently not used, could have major public health benefits that will far outweigh any initial costs.” Speaking to Health Estate Journal Professor Shaman said that, while there remained further research work to do to answer a number of so far unanswered questions, he believed he and his coauthor’s latest research findings could have “potentially significant” implications for the way humidity is controlled and maintained in hospitals. He added: “I cannot really comment with any certainty on the extent to which US hospitals have effectively deployed humidification in ward environments on the basis of its potential impact on influenza virus survival and transmission. However there is little doubt from much of the scientific evidence to date, and our study appears to back this, that if a hospital raises humidity to a sensible level in its wards and operating theatres, especially during a flu pandemic, the virus’s survival and transmission rates will be significantly reduced. “At other times, he adds, “it appears that the best course will be to keep relative and absolute humidity within a ‘sweet spot’ area, certainly at levels at which patients are comfortable. I was also interested in Vapac’s contention that if you can keep the droplets larger they are more likely to fall on tables and other surfaces which can then easily be cleaned, and are less likely to remain aerosolised, and thus be transmitted to other individuals. “What is interesting here is that, to my knowledge, there is very little real understanding of the means by which the flu virus is transmitted, i.e. whether it is more likely to be passed from one person to another via a contact surface, or through the air. More research is needed in this field too, while my colleagues and I are already undertaking further study into the whole issue of humidity’s effect on influenza and other pathogens, both at a laboratory level, and in clinical settings. This is undoubtedly an extremely interesting subject area.”