星空传媒

Hospital acquired infection (hai) is a significant problem in the NHS. Recent estimates suggest that one in 10 patients acquire an infection during their stay in hospital leading to at least 100 000 hospital acquired infections occurring each year. The costs to the NHS alone are a staggering 拢1 billion a year. Yet this is a gross under-estimate of the true costs, as adding in loss of income and the socio-economic impact of the 5000 deaths that hai is the main cause of and the 15 000 deaths where hai is a substantial contributor, could easily take that figure to 拢6-11 billion. A recent case highlighted in the London Evening Standard is typical of the tragic impact. Anthony Westbrook, a 49 year old driving instructor, went in to Ashford Hospital in January suffering from breathing difficulties. Diagnosed with an uncommon blood disorder, he then contracted methicillin resistant staphylococcus aureus (mrsa) and died from a severe haemorrhage.

While the main emphasis of the management response to hai has so far been to push for greater rigour from NHS staff in preventing infection, such as cleaning hands and equipment, this does not solve the whole problem. Despite great advances in infection control practice, the incidence of hai or nosocomial infection has remained static for the past 20 years. Recent research for Hoare Lea Consulting Engineers has demonstrated that while person to person contact remains the biggest cause of hai, ventilation practice and design can have a significant impact on hai incidence.

While the airborne route of infection has long been recognised, with the exception of tuberculosis, the contribution made by airborne pathogens towards nosocomial infection has often been underestimated. Part of the problem is one of communication and definition. There is a distinction made by the NHS and researchers between droplet-spread and droplet nuclei-spread infection which is highly questionable. The medical profession deems contact-spread to include the droplet-spread of infectious agents. Yet droplets can evaporate very quickly and become droplet nuclei. These can also remain suspended for many hours and be carried over long distances by convection currents. In strict physical terms both modes of transport are thus airborne.

Work by Dr Clive Beggs from the University of Leeds, liaising closely with colleagues from the medical faculty, has estimated that infection spread by convection can account for 10-20% of endemic nosocomial infections. Assuming 拢6 billion true costs of hai per annum, the convection spread component may cost 拢600-1200 million per year. If even 10% of that could be prevented by better ventilation practice and design, it would be a significant saving.

The medical profession has naturally sought to control the problem through the use of antibiotics and powerful sterilising chemicals. These in turn have led to endemic problems of drug resistance and chemical sensitivity. Hence wider strategies are needed.

With plans for significant new hospital build and investment in current hospitals in the UK well underway, there is concern that out-of-date design, particularly relating to ventilation, may build in health problems for the future. To understand the scale of the problem, we need to understand how airborne infection spreads.

In hospitals, most of the micro-organisms in the air are generated by the occupants and do not come from outside. The exception is where windows are opened in wards to allow air to come in. In this case, additional risks enter the hospital. Generally the higher the occupancy in the hospital rooms and wards the higher the microbial bioburden in the air. Various air-sampling surveys have shown that the highest counts of potentially infectious organisms occur in laundry-handling areas, though corridors and general wards also showed high levels, particularly in older hospitals. Bed-making has been shown to be a major process for dispersing potential infection.

Taking on the superbugs
The tabloid newspapers have made great play of so-called superbugs which resist drug control. In recent years, drug resistant strains of staphylococcus aureus, including methicillin resistant staphylococcus aureus (mrsa) have become endemic in many hospitals. Although infection with mrsa is generally associated with person to person contact, airborne transmission has occurred in a variety of settings, including operating theatres, intensive care, burns and orthopaedic units. A report on one mrsa outbreak showed that 16% to 31% of the problem was unlikely to have come from person to person contact.

Infections where the airborne route plays an important role include Pseudomonas aeruginosa, coagulase-negative staphylococci, aspergillus fumigatus, the well-known legionella pneumophilia (legionnaires disease) and tuberculosis.

Airborne infectious agents can be transported over long distances by convection currents and bio-aerosols remain suspended in air for long periods. Ventilation has long been seen as the primary means of controlling airborne infection, either through dilution or through isolation using differential pressure control. Poor design, operation or maintenance can lead to its own problems, a point recently highlighted where mechanical ventilation system acted as a conduit for distributing pathogens around buildings. A recent example in a London hospital spread tuberculosis to HIV-positive patients.

Current design standards
The current NHS design standards concentrate on the design of systems serving the critical clinical areas such as isolation suites of operating theatres. Spaces such as waiting rooms and wards receive scant attention and the guidance for these areas are in need of updating.

NHS Health Technical Memorandum (HTM) 2025 is very vague on the subject of ventilation rates for general clinical spaces, simply stating a figure of 10 ac/h for 'treatment rooms'. This is far too high for general ward spaces. US standards are much more prescriptive, giving minimum levels for all parts of a hospital. The basis for this seems uncertain and needs to be examined with a view to matching ventilation rates to infection control requirement.

The best model so far developed to assess the benefits of ventilation is the Wells-Riley equation which relates the probability of infection to infective agent production rate, exposure time and ventilation rate. This is not a perfect model and needs further work to improve predictability, but it is the best tool at our disposal. It can be used at the design and modelling stage to predict the theoretical impact of the outbreak of altering the ventilation rate in the building, allowing a much better balance between costs and health than at present.

Most mechanical ventilation systems in the NHS work on a dilution principle. Hoare Lea has modified the Wells Riley equation to include ventilation effectiveness, enabling the benefits of alternative solutions to dilution ventilation to be compared and contrasted. Yet very few hospitals are actually designed using it, as NHS standards do not make it mandatory.

Any problem which causes 拢6 to 拢11 billion in additional, and to some extent preventable, costs to the NHS and society at large is worthy of serious attention and investment. The airborne route of infection for hai is very significant, yet its role is grossly underestimated by health professionals and management. As such, and apart from operating theatres and acute wards, good ventilation design in order to reduce the opportunity for hai is given low prominence by those making decisions on hospital design and costs.

We think the time has come to quickly review current approaches to hai and give greater prominence to the airborne route of infection. This would include redefining airborne infection to reflect reality, and improving and utilising current design tools for reducing infection. It should also test out design approaches in both hospitals and doctors waiting areas, another area which may easily spread infection to patients. Critically, NHS advice under HTM 2025 is grossly out of date and needs urgent review. It would be a great tragedy if the long overdue upgrading of the NHS estate might be undermined by poor design for ventilation.