Potential for airborne transmission of infection in the waiting areas of healthcare premises: stochastic analysis using a Monte Carlo model

Abstract

BackgroundAlthough many infections that are transmissible from person to person are acquired through direct contact between individuals, a minority, notably pulmonary tuberculosis (TB), measles and influenza are known to be spread by the airborne route. Airborne infections pose a particular threat to susceptible individuals whenever they are placed together with the index case in confined spaces. With this in mind, waiting areas of healthcare facilities present a particular challenge, since large numbers of people, some of whom may have underlying conditions which predispose them to infection, congregate in such spaces and can be exposed to an individual who may be shedding potentially pathogenic microorganisms. It is therefore important to understand the risks posed by infectious individuals in waiting areas, so that interventions can be developed to minimise the spread of airborne infections.MethodA stochastic Monte Carlo model was constructed to analyse the transmission of airborne infection in a hypothetical 132 m3 hospital waiting area in which occupancy levels, waiting times and ventilation rate can all be varied. In the model the Gammaitoni-Nucci equation was utilized to predict probability of susceptible individuals becoming infected. The model was used to assess the risk of transmission of three infectious diseases, TB, influenza and measles. In order to allow for stochasticity a random number generator was applied to the variables in the model and a total of 10000 individual simulations were undertaken. The mean quanta production rates used in the study were 12.7, 100 and 570 per hour for TB, influenza and measles, respectively.ResultsThe results of the study revealed the mean probability of acquiring a TB infection during a 30-minute stay in the waiting area to be negligible (i.e. 0.0034), while that for influenza was an order of magnitude higher at 0.0262. By comparison the mean probability of acquiring a measles infection during the same period was 0.1349. If the duration of the stay was increased to 60 minutes then these values increased to 0.0087, 0.0662 and 0.3094, respectively.ConclusionUnder normal circumstances the risk of acquiring a TB infection during a visit to a hospital waiting area is minimal. Likewise the risks associated with the transmission of influenza, although an order of magnitude greater than those for TB, are relatively small. By comparison, the risks associated with measles are high. While the installation of air disinfection may be beneficial, when seeking to prevent the transmission of airborne viral infection it is important to first minimize waiting times and the number of susceptible individuals present before turning to expensive technological solutions.

Room ventilation and the risk of airborne infection transmission in 3 health care settings within a large teaching hospital

“…Beggs et al 23 estimated the risks of airborne transmission of TB, influenza, and measles in a hypothetical hospital waiting area containing a single infectious individual and found respective mean risks of infection for susceptible persons of 0.3%, 2.6%, and 13.5% for a 30-minute wait and 0.8%, 6.6%, and 30.9% for a 60-minute wait. Despite differences in methodology, the present study and that of Beggs et al 23 highlight the approximate relative risks faced by individuals during the time spent waiting for and during medical consultations. The risk of an individual acquiring influenza appears more likely during the actual consultation, if the room was previously occupied by an infectious individual, than during the period spent in a waiting room based on the limited scenarios modeled in the 2 studies.…”

“…It is important to keep in mind that our risk estimates are expressed as percentages, and although this is a convenient and intuitive metric, it might not translate to a significant absolute number of infections in a room with low occupancy. 19,23 The points at which the existing air exchange rate in the Lung Function Laboratory intersects the curves shown in Figure 1 indicate that increasing air exchange further would provide a negligible reduction to an already very low infection risk. The attendant increase in energy consumption required would be difficult to justify when the size of the room and its typical occupancy of up to 10 persons are considered.…”

Extensive Nosocomial Transmission of Measles Originating in Cruise Ship Passenger, Sardinia, Italy, 2014

“…As described in this outbreak, nosocomial transmission can lead to measles in HCWs, other patients at the hospital, and susceptible hospital visitors. Measles is one of the most contagious viral diseases and patients frequently visit hospitals or health care facilities for diagnosis and management, which might lead to nosocomial transmission if appropriate infection control measures are not immediately instituted ( 8 ). …”

Extremely low risk for acquisition of a respiratory viral infection in the emergency room of a large pediatric hospital during the winter season

“…A likely explanation for these findings is the high baseline risk of respiratory infection in children. In a study using a Monte Carlo stochastic model to analyze the transmission of airborne infections in a hospital waiting area under various occupancy levels, waiting times, and ventilation rates, it was found that the mean probability for acquiring influenza, although relatively small, increased with increasing waiting times and decreasing distance among visitors 12 . Emergency departments in Greek hospitals are typically overcrowded during winter and many parents in our study reported that they moved away from symptomatic patients, a fact that may had an impact on the extremely low rate of transmission of infection.…”