Influence of ceiling fan's speed and direction on efficacy of upperroom, ultraviolet germicidal irradiation: Experimental.

Increasing a ceiling fan's speed from its lowest setting of 61 rpm, which resulted in 0.77 m3/s of airflow, to its highest setting of 176 rpm, which resulted in 2.5 m3/s of airflow, or having the fan blow either upward or downward had no statistically significant effect on the efficacy of upper-room ultraviolet germicidal irradiation (UVGI). This outcome suggests that air circulation due to the ceiling fan was sufficient and that any additional increase would not improve efficacy. Numerous experimental studies on upper-room UVGI in which fans were used to provide air mixing have been published. However, none have quantified the air movement produced by these fans or described their tests in sufficient detail to allow results to be compared to predictions using computational fluid dynamics (CFD). The present work provides the required information. In addition to the usual boundary conditions needed for CFD, we made experimental measurements of UV susceptibility of the microorganisms used in the upper-room UVGI tests. We measured UV susceptibilities for Mycobacterium parafortuitum and Bacillus atrophaeus spores to be 0.074 and 0.018 m2/J, respectively. In a previous publication, we reported the spatial distribution of fluence rate, which is also needed for predicting efficacy from CFD. In a companion paper referred to as Part II, upper-room UVGI efficacy was predicted by both Eulerian and Lagrangian CFD and compared to the experimental results from the present study.

Eggcrate UV: a whole ceiling upper-room ultraviolet germicidal irradiation system for air disinfection in occupied rooms.

A novel whole ceiling upper-room ultraviolet germicidal irradiation (UVGI) system [eggcrate ultraviolet (UV)] has been developed that incorporates open-cell 'eggcrate'-suspended ceiling panels and bare UV lamps with a ceiling fan. Upper-room UVGI is more effective for air disinfection than mechanical ventilation at much lower installation and operating costs. Conventional upper-room UVGI fixtures employ multiple tightly spaced horizontal louvers to confine UV to the upper-room. These louvered fixtures protect occupants in the lower-room from UV-induced eye and skin irritation, but at a major cost to fixture efficiency. Using a lamp and ballast from a conventional upper-room UVGI fixture in the eggcrate UV system, the germicidal efficacy was markedly improved even though the UV radiation emitted by the lamp was unchanged. This fundamental change in the application of upper-room UVGI air disinfection should permit wider, more effective application of UVGI globally to reduce the spread of airborne infection.

Risk of indoor airborne infection transmission estimated from carbon dioxide concentration.

The Wells-Riley equation, which is used to model the risk of indoor airborne transmission of infectious diseases such as tuberculosis, is sometimes problematic because it assumes steady-state conditions and requires measurement of outdoor air supply rates, which are frequently difficult to measure and often vary with time. We derive an alternative equation that avoids these problems by determining the fraction of inhaled air that has been exhaled previously by someone in the building (rebreathed fraction) using CO2 concentration as a marker for exhaled-breath exposure. We also derive a non-steady-state version of the Wells-Riley equation which is especially useful in poorly ventilated environments when outdoor air supply rates can be assumed constant. Finally, we derive the relationship between the average number of secondary cases infected by each primary case in a building and exposure to exhaled breath and demonstrate that there is likely to be an achievable critical rebreathed fraction of indoor air below which airborne propagation of common respiratory infections and influenza will not occur.

Predicting the ultraviolet radiation distribution in a room with multilouvered germicidal fixtures.

Irradiating the upper part of a room with 254-nm ultraviolet (UV) radiation from a low-pressure mercury discharge lamp has the potential to be a relatively inexpensive method to reduce transmission of airborne infectious diseases such as tuberculosis. To protect occupants in the lower part of a room from radiation, multilouvered UV germicidal fixtures producing a horizontal, collimated beam are often used, particularly in rooms having a normal ceiling height. Knowledge of the fixture's emission characteristics and the airflow field are needed to estimate the UV dose to airborne microorganisms and assess the fixture's overall effectiveness in controlling disease transmission. In this article, a model is developed to predict the UV fluence rate at any location in the upper room for ceiling-mounted, multilouvered, pendant-type fixtures, which provide 360-degree emission in the horizontal plane. The model also predicts total UV power emitted by the fixture, which is the best single-number effectiveness index for comparing multilouvered UV germicidal fixtures. Model predictions compared favorably with laboratory and field measurements.

Optimization of filtration for reduction of lung dose from Rn decay products: Part I--Theoretical.

A theoretical model was developed for the optimization of filter characteristics that would minimize the dose from the inhalation of Rn decay products. Modified forms of the Jacobi-Porstendorfer room model and the Jacobi-Eisfeld lung dose model were chosen for use in the mathematical simulation. Optimized parameters of the filter were the thickness, solidity, and fiber diameter. For purposes of the calculations, the room dimensions, air exchange rate, particle-size distribution and concentration, and the Rn concentration were specified. The resulting computer-aided optimal design was a thin filter (the minimum thickness used in the computer model was 0.1 mm) having low solidity (the minimum solidity used was 0.5%) and large diameter fibers (the maximum diameter used was 100 microns). The simulation implies that a significant reduction in the dose rate can be achieved using a well-designed recirculating filter system. The theoretical model, using the assumption of ideal mixing, predicts an 80% reduction in the dose rate, although inherent in this assumption is the movement of 230 room volumes per hour through the fan.

Optimization of filtration for the reduction of lung dose from Rn decay products: Part II--Experimental.

Research was performed to determine the validity of a model developed to theoretically predict the optimal characteristics of a recirculating filter system for minimizing the lung dose to a person breathing airborne Rn progeny. Four designs, each with different filter thicknesses, solidities, and fiber diameters, were tested to evaluate the accuracy of the model over a range of parameters. Increasing thicknesses were then tested for the most effective filter design to provide a more definitive comparison of experimental data and model predictions for this key parameter. The experimental data supported the conclusion that the most effective design was a thin filter of low solidity composed of coarse fibers. Although the maximum reduction in the dose-equivalent rate observed in these experiments was 50%, this was largely due to constraints on the experimental arrangements. With properly constructed filter units, much better removal efficiencies can undoubtedly be achieved.

Particulate and nicotine sampling in public facilities and offices.

The purpose of this study was to characterize and measure indoor air quality in public facilities and office buildings. The pollutants of interest were particulate matter smaller than 2.5 microns in diameter, PM-2.5, and environmental tobacco smoke (ETS). Integrated PM-2.5 samples were taken on Teflon membrane filters using Harvard Aerosol Impactors as a pre-size selector. Filters were analyzed by gravimetric analysis. Nicotine, which was used as a marker for ETS, was collected on sodium-bisulfate-impregnanted, glass-fiber filters and was analyzed by gas chromatography. Twenty-one structures were monitored in Metropolitan Boston. Measured particle concentrations ranged from 6.0 micrograms/m3 to about 550 micrograms/m3. Nicotine concentrations were as high as 26 micrograms/m3 in a designated smoking room. Real-time measurements were also taken using two types of nephelometers; a Handheld Aerosol Monitor (HAM) and a Miniature Real-Time Aerosol Monitor (MINIRAM). Short-term field measurements with these instruments correlated better with the integrated PM-2.5 concentrations in smoking locations than with concentrations in non-smoking areas.

Effective removal of airborne 222Rn decay products inside buildings.

Comparisons were made of the effectiveness of various indoor air treatment methods in reducing the lung dose due to inhalation of 222Rn decay products. The comparisons were based upon measurements of the total steady-state concentrations of 218Po, 214Pb and 214Bi, and the concentrations of these nuclides not attached to particles. These measurements, which were made inside a 78-m3 room before and after air treatment, were used along with a state-of-the art lung dose model to predict reductions in the dose to the radiosensitive bronchial tissues. Results suggest that flow-through air-cleaning methods, such as filtration and electrostatic precipitation, although effective in reducing total potential alpha energy concentration, cause a greater quantity of airborne potential alpha energy to be unattached to particles. This may result in a substantial increase in the dose to bronchial tissues. The optimal form of air treatment appears to be a combination of nonuniform positive space charge generated by an ion generator and enhanced convection from a fan. This combination of air treatment gave reductions in the mean dose to the bronchial tissues of up to 87%.

Surface deposition of 222Rn decay products with and without enhanced air motion.

The effectiveness of fan-induced air motion in reducing airborne activities of short-lived 222Rn decay products was evaluated in a 78.5-m3 chamber. Observed reductions were as high as 50% for 218Po (RaA), 79% for 214Pb (RaB), and 86% for 214Bi (RaC). Activity measurements of these nuclides on chamber and fan surfaces, along with airborne activities, were used to calculate material balances. Greater than about 90% of deposited activity was found on chamber surfaces, although areal activity densities were higher on fan surfaces. Deposition velocities for decay products not attached to particles were 2.3 mm s-1 when no fans were in operation and 9.2 to 13 mm s-1 when fans were used. Mean boundary layer thicknesses for unattached decay products were estimated to be about four times the recoil distance of a 214Pb atom when no fans were used and about equal to the recoil distance when fans were used.

Effect of plateout, air motion and dust removal on radon decay product concentration in a simulated residence.

The effectiveness of increased air motion and dust removal in reducing radon decay product concentration in residences subject to radon intrusion was evaluated in a 78-m3 room under steady-state conditions for air infiltration rates between 0.2 and 0.9 air changes per hour. Room-size, portable electrostatic precipitators and high-efficiency fibrous filters were tested as typical residential air cleaning devices; a portable box fan and a ceiling fan were employed as typical residential air movers. Reductions in working levels of 40-90% were found. The fate of radon decay products, with and without mixing fans, was determined by direct measurement. When mixing fans were used, most of the nonairborne potential alpha-energy was plated out on the room surfaces; less than 10% was deposited on the fan blades or housing. Results were compared to a mathematical model based on well-mixed room air, and good agreement was obtained.