Pathogen Control Engineering Institute

The use of Upper Room Ultra-Violet light for Infection Control

The high world-wide prevalence of tuberculosis and increased concerns due to the complications of HIV/AIDS and multi-drug resistant strains, coupled with the emerging threat posed by bio terrorism have prompted renewed interest in the use of ultraviolet germicidal irradiation (UVGI) devices to disinfect indoor air. UV-C light at a wavelength close to 254 nm can cause lethal damage to the DNA of microorganisms. By mounting UV-C lamps in shielded devices located above head-height, it is possible to use the airflow in the room to transport bioaerosols through a UV field, reducing the bioburden in the air and hence the risk of infection to occupants of the room.

Objectives

The aim of this study was to investigate how the performance of UVGI devices is dependent on the species of microorganism, the climatic conditions and the ventilation in the room

Methodologies

The study used three key techniques to investigate the effectiveness of UVGI disinfection

Bench scale lab tests

A custom built single-pass test rig (link) was used to expose aerosolised microorganisms to a controlled dose of UV-C irradiation under a range of humidity conditions. A Collision nebuliser was used to aerosolise microorganisms suspended in distilled water through a UV chamber at a constant rate. Sampling the microorganisms after the chamber using a 6-stage Anderson sampler for a range of different UV doses, generated quantitative data on the survival of microorganisms which informed the other areas of the study.

Room scale experiments

Full-scale trials using commercially available UV devices were carried out in the Aerobiology test chamber (link). Microrganisms were nebulised into the centre of the room through a tube and the air was sampled close to the ventilation extract using the Anderson sampler. Experiments were carried out both with and without the UV devices to establish the level of bioaerosol inactivation in the presence of the UV devices. In each case 10 replicate samples were taken over a period of 2-3 hours to establish an average bioaerosol concentration. The experiments were carried out at several different ventilation rates to establish the device performance in each case.

Computational modelling

A range of modelling techniques were used to inform and understand the behaviour in the experimental trials as well as to demonstrate how the devices should be used. Ventilation models were used to gain a quick insight into the potential for disinfection and the likely effect of the ventilation system. A biological decay model was incorporated into CFD simulations that enabled the disinfection process to be modelled in detail showing the concentration of viable microorganisms to be evaluated throughout the room.

Key Results

Susceptibility data

Typical data from a single-pass UV susceptibility experiment are shown in figure 1, demonstrating the variation in susceptibility with microorganism species. The results of the study also indicated that increasing the relative humidity leads to an observed decrease in the UV susceptibility of aerosolised gram negative bacteria, with the degree to which the UV susceptibility is affected dependent on species. The most likely explanation for the variability with humidity is that of environmental stress resulting from a combination of the nebulization process and desiccation/dehydration under lower relative humidity conditions 1

Figure 1: UV susceptibility data from the single-pass test rig.

Room performance

Table 1 shows experimentally acquired survival data for Serratia marcesens in the presence of two wall mounted UV devices at air change rates of 6 and 9 AC/h. In each case the results are presented in terms of a survival fraction compared to the bioaerosol concentration in the room without the UV devices at the same ventilation rate. The standard deviation is also presented calculated from 10 experimental replicates without UV and 10 replicates with UV irradiation. The results demonstrate that UVGI devices have a greater relative impact at lower ventilation rates 2

Ventilation rate (AC/h)	Relative Humidity (%)	Survival fraction
6	48.70 (SD 2.34)	0.572 (SD 0.20)
9	48.16 (SD 1.63)	0.685 (SD 0.06)

Table 1: Survival data for Serratia marcesens in the presence of two upper-room UVGI devices

The results from CFD simulations in Figure 2 show the predicted bioaerosol distribution in the chamber with and without UV irradiation 3,4 . It can be seen that although the UV devices significantly reduce the bioaerosol concentration through the bulk of the room, close to the source in the centre of the room, the UV has little impact, with a plume of high concentration apparent in both cases.

Figure 2: Bioaerosol distribution in test chamber at 6 AC/h, predicted from CFD simulations.

Effect of ventilation system

Ventilation models validated against CFD simulations enable the effect of the ventilation system to be easily demonstrated. For the simplified room layout shown in figure 3, ventilation models can be used to examine the average bioaerosol concentration in different zones within the room with the addition of UVGI devices, as shown by figure 4. Similar results can be produced for different ventilation layouts to demonstrate the effect of the air mixing on the UVGI effectiveness 4,5 .

Figure 3: Simplified room layout for ventilation modelling

Figure 4: Typical results from ventilation models showing effect of ventilation rate on bioaerosol concentrations