Relationship between healthcare worker surface contacts, care type and hand hygiene: an observational study in a single-bed hospital ward.

This study quantifies the relationship between hand hygiene and the frequency with which healthcare workers (HCWs) touch surfaces in patient rooms. Surface contacts and hand hygiene were recorded in a single-bed UK hospital ward for six care types. Surface contacts often formed non-random patterns, but hygiene before or after patient contact depends significantly on care type (P=0.001). The likelihood of hygiene correlated with the number of surface contacts (95% confidence interval 1.1-5.8, P=0.002), but not with time spent in the room. This highlights that a potential subconscious need for hand hygiene may have developed in HCWs, which may support and help focus future hygiene education programmes.

Modeling environmental contamination in hospital single- and four-bed rooms.

UNLABELLED: Aerial dispersion of pathogens is recognized as a potential transmission route for hospital acquired infections; however, little is known about the link between healthcare worker (HCW) contacts' with contaminated surfaces, the transmission of infections and hospital room design. We combine computational fluid dynamics (CFD) simulations of bioaerosol deposition with a validated probabilistic HCW-surface contact model to estimate the relative quantity of pathogens accrued on hands during six types of care procedures in two room types. Results demonstrate that care type is most influential (P < 0.001), followed by the number of surface contacts (P < 0.001) and the distribution of surface pathogens (P = 0.05). Highest hand contamination was predicted during Personal care despite the highest levels of hand hygiene. Ventilation rates of 6 ac/h vs. 4 ac/h showed only minor reductions in predicted hand colonization. Pathogens accrued on hands decreased monotonically after patient care in single rooms due to the physical barrier of bioaerosol transmission between rooms and subsequent hand sanitation. Conversely, contamination was predicted to increase during contact with patients in four-bed rooms due to spatial spread of pathogens. Location of the infectious patient with respect to ventilation played a key role in determining pathogen loadings (P = 0.05). PRACTICAL IMPLICATIONS: We present the first quantitative model predicting the surface contacts by HCW and the subsequent accretion of pathogenic material as they perform standard patient care. This model indicates that single rooms may significantly reduce the risk of cross-contamination due to indirect infection transmission. Not all care types pose the same risks to patients, and housekeeping performed by HCWs may be an important contribution in the transmission of pathogens between patients. Ventilation rates and positioning of infectious patients within four-bed rooms can mitigate the accretion of pathogens, whereby reducing the risk of missed hand hygiene opportunities. The model provides a tool to quantitatively evaluate the influence of hospital room design on infection risk.

Effects of turbulence modelling on prediction of flow characteristics in a bench-scale anaerobic gas-lift digester.

Flow in a gas-lift digester with a central draft-tube was investigated using computational fluid dynamics (CFD) and different turbulence closure models. The k-ω Shear-Stress-Transport (SST), Renormalization-Group (RNG) k-∊, Linear Reynolds-Stress-Model (RSM) and Transition-SST models were tested for a gas-lift loop reactor under Newtonian flow conditions validated against published experimental work. The results identify that flow predictions within the reactor (where flow is transitional) are particularly sensitive to the turbulence model implemented; the Transition-SST model was found to be the most robust for capturing mixing behaviour and predicting separation reliably. Therefore, Transition-SST is recommended over k-∊ models for use in comparable mixing problems. A comparison of results obtained using multiphase Euler-Lagrange and singlephase approaches are presented. The results support the validity of the singlephase modelling assumptions in obtaining reliable predictions of the reactor flow. Solver independence of results was verified by comparing two independent finite-volume solvers (Fluent-13.0sp2 and OpenFOAM-2.0.1).

CFD simulation of airborne pathogen transport due to human activities.

Computational Fluid Dynamics (CFD) is an increasingly popular tool for studying the impact of design interventions on the transport of infectious microorganisms. While much of the focus is on respiratory infections, there is substantial evidence that certain pathogens, such as those which colonise the skin, can be released into, and transported through the air through routine activities. In these situations the bacteria is released over a volume of space, with different intensities and locations varying in time rather than being released at a single point. This paper considers the application of CFD modelling to the evaluation of risk from this type of bioaerosol generation. An experimental validation study provides a direct comparison between CFD simulations and bioaerosol distribution, showing that passive scalar and particle tracking approaches are both appropriate for small particle bioaerosols. The study introduces a zonal source, which aims to represent the time averaged release of bacteria from an activity within a zone around the entire location the release takes place. This approach is shown to perform well when validated numerically though comparison with the time averaged dispersion patterns from a transient source. However, the ability of a point source to represent such dispersion is dependent on airflow regime. The applicability of the model is demonstrated using a simulation of an isolation room representing the release of bacteria from bedmaking.

Health risks in wastewater irrigation: comparing estimates from quantitative microbial risk analyses and epidemiological studies.

The combination of standard quantitative microbial risk analysis (QMRA) techniques and 10,000-trial Monte Carlo risk simulations was used to estimate the human health risks associated with the use of wastewater for unrestricted and restricted crop irrigation. A risk of rotavirus infection of 10(-2) per person per year (pppy) was used as the reference level of acceptable risk. Using the model scenario of involuntary soil ingestion for restricted irrigation, the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or =10(6) Escherichia coli per 100ml and when local agricultural practices are highly mechanised. For labour-intensive agriculture the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or = 10(5) E. coli per 100ml; however, the wastewater quality should be < or = 10(4) E. coli per 100ml when children under 15 are exposed. With the model scenario of lettuce consumption for unrestricted irrigation, the use of wastewaters containing < or =10(4) E. coli per 100ml results in a rotavirus infection risk of approximately 10(-2) pppy; however, again based on epidemiological evidence from Mexico, the current WHO guideline level of < or =1,000 E. coli per 100ml should be retained for root crops eaten raw.

Modelling the transmission of airborne infections in enclosed spaces.

The Wells-Riley equation for modelling airborne infection in indoor environments is incorporated into an SEIR epidemic model with a short incubation period to simulate the transmission dynamics of airborne infectious diseases in ventilated rooms. The model enables the effect of environmental factors such as the ventilation rate and the room occupancy to be examined, and allows the long-term impact of infection control measures to be assessed. A theoretical parametric study is carried out to demonstrate how changes to both the physical environment and infection control procedures may potentially limit the spread of short-incubation-period airborne infections in indoor environments such as hospitals.

Escherichia coli removal in waste stabilization ponds: a comparison of modern and classical designs.

Two PC-based waste stabilization pond design procedures, based on parameter uncertainty and 10,000-trial Monte Carlo simulations, were developed for a series of anaerobic, facultative and maturation ponds to produce < or = 1000 E. coli per 100 ml for both 50% and 95% compliance. One procedure was based on the classical Marais equations and the other on the modern von Sperling equations. For the range of parameter variations selected the classical design procedure required less land area and had a shorter hydraulic retention time than the modern design procedure. For both procedures the design for 90% compliance required substantially more land and a longer retention time than the design for 50% compliance. Regulators and designers should seek a balance between system reliability (as set by the percentage compliance specified or adopted) and system costs, especially (but not only) in developing countries. It is recommended that new waste stabilization pond (WSP) systems be designed for compliance with a given E. coli effluent requirement by the classical procedure and that existing overloaded WSP systems be upgraded using the modern procedure.

The transmission of tuberculosis in confined spaces: an analytical review of alternative epidemiological models.

Tuberculosis (TB) is a disease that is closely associated with poverty, with transmission occurring in situations where infected persons are in close contact with others in confined spaces. While it is well recognised that overcrowding increases the risk of transmission, this increased risk has not been quantified and the relationship between overcrowding and duration of exposure is not well understood. This paper analyses three epidemiological models that have been used to predict the transmission of airborne disease in confined spaces: the Mass Action model, Riley, Murphy and Riley's model and Gammaitoni and Nucci's model. A study is presented to demonstrate the range of applicability of each model and show how they can be applied to the transmission of both TB and diseases with short incubation periods such as measles. Gammiatoni and Nucci's generalised formulation is shown to be the most suitable for modelling airborne transmission in ventilated spaces, and it is subsequently used in a parametric study to evaluate the effect of physical and environmental factors on the rate of disease transmission. The paper also presents reported quanta production data for several TB outbreaks and demonstrates that the greatest risk of TB infection is during clinical procedures that produce large quantities of aerosol, such as bronchoscopy or intubation.

Susceptibility of Burkholderia cepacia and other pathogens of importance in cystic fibrosis to u.v. light.

To investigate the potential usefulness of u.v. germicidal irradiation (UVGI) in preventing the spread of Burkholderia cepacia, an important pathogen in cystic fibrosis (CF), the in-vitro susceptibility of B. cepacia to UVGI was determined. Five strains were exposed to UVGI from a 7.2-W source. Burkholderia cepacia was less susceptible to UVGI than other important CF-related pathogens, namely Staphylococcus aureus and Pseudomonas aeruginosa, but was more susceptible than Stenotrophomonas maltophilia. No strain of B. cepacia survived longer than an 8 s exposure to UVGI, with doses required to achieve 1 log reduction in bacterial numbers ranging from 28.3 to 57.5 J m(-2).

The resurgence of tuberculosis in the tropics. An engineering approach to the control of Mycobacterium tuberculosis and other airborne pathogens: a UK hospital based pilot study.

The world-wide occurrence of tuberculosis (TB) is very high, and in many parts of the world prevalence has reached epidemic proportions. While the WHO's global 'directly observed therapy short-course' (DOTS) programme has yielded some notable successes, it has reached only 12% of the world's TB cases (1996 data). This suggests that the use of drug therapy alone is not enough to solve the global TB problem and that prevention using public health engineering techniques may provide a complementary solution. There are a number of engineering control strategies, such as the use of ultraviolet germicidal irradiation (UVGI) and advanced ventilation techniques, which can be used to combat the spread of Mycobacterium tuberculosis and other airborne pathogens. This paper describes a pilot study currently being undertaken at the Leeds General Infirmary in the UK, which is investigating the use of UVGI to disinfect air in ward spaces.