Airborne pathogens diffusion: A comparison between tracer gas and pigmented aerosols for indoor environment analysis

The evaluation of airborne pathogens diffusion is a crucial practice in preventing airborne diseases like COVID-19, especially in indoor environments. Through this transmission route, pathogens can be carried by droplets, droplet nuclei and aerosols and be conveyed over long distances. Therefore, understanding their diffusion is vital for prevention and curbing disease transmission. There are different techniques used for this purpose, and one of the most common is the utilization of tracer gas, however, it has limitations such as the difference in size between the gas molecules and the respiratory droplets, as well as its incapability to take into account evaporation. For this reason, a new method for evaluating the diffusion of respiratory droplets has been developed. This approach involves the use of an ultrasonic emitter to release and disperse pigmented aerosols, and a colorimeter for the following quantitative evaluation. A comparison with the tracer gas technique has been carried out, showing for the pigmented aerosols methodology a response that is dependent on different relative humidity conditions, while there is no clear difference in the dispersion of tracer gas at high or low humidity.

Tempo-spatial infection risk assessment of airborne virus via CO2 concentration field monitoring in built environment

The aerosol transmission was academically recognized as a possible transmission route of Coronavirus disease 2019 (COVID-19). We established an approach to assess the indoor tempo-spatial airborne-disease infection risks through aerosol transmission via real-time CO₂ field measurement and occupancy monitoring. Compared to former studies, the proposed method can evaluate real-time airborne disease infection risks through aerosol transmission routes. The approach was utilized in a university office. The accumulated infection risk was calculated for three occupants with practical working schedules (from occupancy recording) and one hypothesis occupant with a typical working schedule. COVID-19 was used as an example. Results demonstrated that the individual infection risks diversified with different dwell times and working places in the office. For the three occupants with a practical working schedule, their 3-day accumulated infection risks were respectively 0.050%, 0.035%, 0.027% and 0.041% due to 11.6, 9.0 and 13.8 h exposure with an initial infector percentage of 1%. The results demonstrate that location and dwell time are both important factors influencing the infection risk of certain occupant in built environment, whereas existing literature seldom took these two points into consideration simultaneously. On the contrary, our proposed approach treated the infection risks as place-by-place, time-by-time and person-by-person diversified in the built environment. The risk assessment results can provide early warning for building occupants and contribute to the transmission control of air-borne disease.

A new method for air exchange efficiency assessment including natural and mixed mode ventilation

The COVID-19 health crisis highlighted the correlation between air exchange efficiency and virus airborne transmission. Air exchange efficiency is a performance index able to characterize ventilation effectiveness in buildings. Some standards, such as ASHRAE 129, clearly define assessment procedures of air exchange efficiency for mechanical ventilation, adopting tracer gas techniques. However, standardized procedures are based on measurements at the exhaust and cannot be adopted for natural and mixed mode ventilation strategies. In the '80s, Sandberg suggested that tracer gas decay technique enables to measure simultaneously the nominal time constant (through air change rate measurements) and the mean age of air in several points of the ventilated zone. This paper aims to present practical issues and uncertainty analysis related to the implementation of this approach, in a new commissioning protocol. For this purpose, we compare the new procedure, based on Sandberg's observation, with the ASHRAE 129 protocol for mechanical ventilation. Results coming from field campaigns show that the difference between air exchange efficiency values obtained using ASHRAE 129 protocol (51.8%) and the new procedure (47.4%) are usually negligible in low airflow rate, considering an average uncertainty of ± 7.0%. Results show that the procedure is robust and that it is technically possible to implement it to natural and mixed-mode ventilation.

A Field Study of Longwall Mine Ventilation Using Tracer Gas in a Trona Mine

A ventilation research study was conducted by the National Institute for Occupational Safety and Health and a cooperating trona mine in the Green River basin of Wyoming, USA. The mine operation uses the longwall mining method in trona bed 17, a commonly mined unit in the region. The longwall face length is 228 m (750 ft), and caving on the face occurred up to the back of the longwall shields. The mine is ventilated using a main blowing fan and a bleeder shaft. For this study, sulfur hexafluoride (SF6) tracer gas was released in two separate monitoring experiments. For the first experiment, tracer gas was released on the face, this test focused on airflow along the longwall face of the active panel. Face test showed the airflow patterns to be more complex than just head-to-tail flow in the main ventilation air stream on the active panel. For the second experiment, tracer gas was released 2 crosscuts inby the face on the headgate side, this test focused on gas transport in the mined-out portion of the same active panel. Gob test showed a pathway of movement through the front of the active panel gob that moved outby from the tailgate corner. The primary pathway of tracer gas movement in the active panel gob was towards the headgate and tailgate bleeders and out of a bleeder shaft. The rate of movement towards the back of the gob was measured to be 0.19 m/s (37 fpm).

Face Ventilation on a Bleederless Longwall Panel

A ventilation study using tracer gas was conducted at a western US coal mine. The objective of the study was to evaluate the movement of longwall face air exchanges between the face and worked-out area and to document the presence or absence of face airflow pathways between these locations. The mine operator uses a bleederless longwall ventilation system with a back return and a blowing mine ventilation system. The study was conducted on an active panel and included both underground and surface monitoring sites. The study used sulfur hexafluoride (SF₆) released as a slug on the longwall face and in the front of the gob inby the face. The velocity of the tracer gas movement in the gob was 0.019 m/s (3.7 fpm). The rate of movement for the overall tracer gas slug averaged about 0.0091 m/s (1.8 fpm). A separate tracer gas test initiated with the release of SF₆ into the legs of the first shield showed the existence of more than one pathway of face air in the general direction from the headgate towards the tailgate corner. Maintaining adequate ventilation air on longwall faces is important for worker safety and for the dilution of methane emitted from the face and caved gob. A more detailed characterization of longwall system air and gas movement allows a mine to better assess its ventilation design for controlling gas on the face and in the gob.

On-site and ground-based remote sensing measurements of methane emissions from four biogas plants: A comparison study

Methods for quantifying methane (CH₄) emissions from biogas plants are needed, in order to ensure that emissions are within acceptable levels and to identify options for emission mitigation. Two emission measuring approaches were used at four biogas plants: an on-site approach, whereby emission sources were identified and subsequently quantified one at a time, and a ground-based remote sensing approach, which was applied to measure total CH₄ emissions. The emissions were between 5.5 and 13.5 kg CH₄ h^-1 from the four plants, measured using ground-based remote sensing. Even though the measurements were performed on the same days at each facility, the sum of on-site emission rates varied between the remote sensing measurements (up to ∼100%). Several factors may have caused this difference: emission sources not measured using an on-site approach and short-time emission variation. On-site measurements showed that the majority of the emissions often occurred from just a few sources.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part B: Effect of source location

The pollutant behavior in and around a naturally ventilated building requires to be investigated quantitatively as the growing concern on air quality within the built environment. The objective of the present study is to further investigate the wind induced inter-flat pollutant transmission and cross contamination routes in typical buildings in Shanghai. In this paper, a set of experiments was carried out in a boundary layer wind tunnel using a 1:30 reduced scale model that represented the typical configuration of rectangular multi-storey residential buildings. Sulfur hexafluoride (SF6) was employed as a tracer gas in the wind tunnel tests. Two natural ventilation modes, single-sided ventilation and cross ventilation were considered. The conditions under prevailing wind direction with different source locations on the windward side were compared. The pressure coefficients on all of the building façades and tracer gas concentration distributions were monitored and analysed. The experimental results elucidated that contaminant released from windward units could spread vertically and horizontally to other units on the source façade and downstream units. The source location was a significant influence factor on the pollutant concentration in various units. In the single-sided ventilated building, the infected risks of leeward units were even higher than those in some windward units. In the cross ventilated building, the vertical transmission could be suppressed and the horizontal transmission was reinforced. The study is helpful for further understanding of the inter-flat airborne transmission within an isolated building.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part A: Effect of wind direction

The inter-flat dispersion of hazardous air pollutants in residential built environment has become a growing concern, especially in crowed urban areas. The purpose of present study is to investigate the wind induced air pollutant transmission and cross contamination routes in typical buildings. In this paper, a series of experiments was carried out in a boundary layer wind tunnel using a 1:30 scaled model that represented the typical configuration of rectangular multi-storey residential buildings in Shanghai. Sulfur hexafluoride (SF₆) was employed as tracer gas in the wind tunnel tests. The conditions under two ventilation modes, i.e. single-sided natural ventilation and cross natural ventilation, were compared. The tracer gas concentration distributions under four approaching wind angles were monitored and analyzed. Computational Fluid Dynamics (CFD) method was adopted to assist in analyzing airflow patterns. The experiment results elucidated that in the two ventilation scenarios, both of the vertical and horizontal inter-flat airborne transmission could proceed. The wind direction played a key role on the pollutant concentration distribution. Compared with the single-sided ventilation mode, cross ventilation could weaken the air pollutant dispersion along the vertical direction when the contamination source was on the windward or on the leeward unit. When the wind blowing parallelly to the source unit window, namely the source room was on the sideward, cross ventilation would not suppress the vertical transport on one hand, but reinforce the horizontal transmission on the other hand. The study is helpful for the analysis of infection risk of respiratory diseases in the residential buildings.

Wind tunnel tests of inter-flat pollutant transmission characteristics in a rectangular multi-storey residential building, part A: Effect of wind direction

The inter-flat dispersion of hazardous air pollutants in residential built environment has become a growing concern, especially in crowed urban areas. The purpose of present study is to investigate the wind induced air pollutant transmission and cross contamination routes in typical buildings. In this paper, a series of experiments was carried out in a boundary layer wind tunnel using a 1:30 scaled model that represented the typical configuration of rectangular multi-storey residential buildings in Shanghai. Sulfur hexafluoride (SF₆) was employed as tracer gas in the wind tunnel tests. The conditions under two ventilation modes, i.e. single-sided natural ventilation and cross natural ventilation, were compared. The tracer gas concentration distributions under four approaching wind angles were monitored and analyzed. Computational Fluid Dynamics (CFD) method was adopted to assist in analyzing airflow patterns. The experiment results elucidated that in the two ventilation scenarios, both of the vertical and horizontal inter-flat airborne transmission could proceed. The wind direction played a key role on the pollutant concentration distribution. Compared with the single-sided ventilation mode, cross ventilation could weaken the air pollutant dispersion along the vertical direction when the contamination source was on the windward or on the leeward unit. When the wind blowing parallelly to the source unit window, namely the source room was on the sideward, cross ventilation would not suppress the vertical transport on one hand, but reinforce the horizontal transmission on the other hand. The study is helpful for the analysis of infection risk of respiratory diseases in the residential buildings.

On-site measurement of tracer gas transmission between horizontal adjacent flats in residential building and cross-infection risk assessment

Airborne transmission is a main spread mode of respiratory infectious diseases, whose frequent epidemic has brought serious social burden. Identifying possible routes of the airborne transmission and predicting the potential infection risk are meaningful for infectious disease control. In the present study, an internal spread route between horizontal adjacent flats induced by air infiltration was investigated. On-site measurements were conducted, and tracer gas technique was employed. Two measurement scenarios, closed window mode and open window mode, were compared. Using the calculated air change rate and mass fraction, the cross-infection risk was estimated using the Wells-Riley model. It found that tracer gas concentrations in receptor rooms are one order lower than the source room, and the infection risks are also one order lower. Opening windows results in larger air change rate on the one hand, but higher mass fraction on the other hand. Higher mass fraction not necessarily results in higher infection risk as the pathogen concentration in the source room is reduced by the higher air change rate. In the present study, opening windows could significantly reduce the infection risk of the index room but slightly reduce the risks in receptor rooms. The mass fraction of air originated from the index room to the receptor units could be 0.28 and the relative cross-infection risk through the internal transmission route could be 9%, which are higher than the external spread through single-sided window flush. The study implicates that the horizontal transmission route induced by air infiltration should not be underestimated.

Estimation of bias with the single-zone assumption in measurement of residential air exchange using the perfluorocarbon tracer gas method

Residential air exchange rates (AERs) are vital in understanding the temporal and spatial drivers of indoor air quality (IAQ). Several methods to quantify AERs have been used in IAQ research, often with the assumption that the home is a single, well-mixed air zone. Since 2005, Health Canada has conducted IAQ studies across Canada in which AERs were measured using the perfluorocarbon tracer (PFT) gas method. Emitters and detectors of a single PFT gas were placed on the main floor to estimate a single-zone AER (AER(1z)). In three of these studies, a second set of emitters and detectors were deployed in the basement or second floor in approximately 10% of homes for a two-zone AER estimate (AER(2z)). In total, 287 daily pairs of AER(2z) and AER(1z) estimates were made from 35 homes across three cities. In 87% of the cases, AER(2z) was higher than AER(1z). Overall, the AER(1z) estimates underestimated AER(2z) by approximately 16% (IQR: 5-32%). This underestimate occurred in all cities and seasons and varied in magnitude seasonally, between homes, and daily, indicating that when measuring residential air exchange using a single PFT gas, the assumption of a single well-mixed air zone very likely results in an under prediction of the AER.

PRACTICAL IMPLICATIONS

The results of this study suggest that the long-standing assumption that a home represents a single well-mixed air zone may result in a substantial negative bias in air exchange estimates. Indoor air quality professionals should take this finding into consideration when developing study designs or making decisions related to the recommendation and installation of residential ventilation systems.

The airborne transmission of infection between flats in high-rise residential buildings: A review

The inter-flat airborne cross-transmission driven by single-sided natural ventilation has been identified recently in high-rise residential buildings, where most people live now in densely populated areas, and is one of the most complex and least understood transport routes. Given potential risks of infection during the outbreak of severe infectious diseases, the need for a full understanding of its mechanism and protective measures within the field of epidemiology and engineering becomes pressing. This review paper considers progress achieved in existing studies of the concerned issue regarding different research priorities. Considerable progress in observing and modeling the inter-flat transmission and dispersion under either buoyancy- or wind-dominated conditions has been made, while fully understanding the combined buoyancy and wind effects is not yet possible. Many methods, including on-site measurements, wind tunnel tests and numerical simulations, have contributed to the research development, despite some deficiencies of each method. Although the inter-flat transmission and dispersion characteristics can be demonstrated and quantified in a time-averaged sense to some extent, there are still unanswered questions at a fundamental level about transient dispersion process and thermal boundary conditions, calling for further studies with more advanced models for simulations and more sound experiments for validations.

The airborne transmission of infection between flats in high-rise residential buildings: A review

The inter-flat airborne cross-transmission driven by single-sided natural ventilation has been identified recently in high-rise residential buildings, where most people live now in densely populated areas, and is one of the most complex and least understood transport routes. Given potential risks of infection during the outbreak of severe infectious diseases, the need for a full understanding of its mechanism and protective measures within the field of epidemiology and engineering becomes pressing. This review paper considers progress achieved in existing studies of the concerned issue regarding different research priorities. Considerable progress in observing and modeling the inter-flat transmission and dispersion under either buoyancy- or wind-dominated conditions has been made, while fully understanding the combined buoyancy and wind effects is not yet possible. Many methods, including on-site measurements, wind tunnel tests and numerical simulations, have contributed to the research development, despite some deficiencies of each method. Although the inter-flat transmission and dispersion characteristics can be demonstrated and quantified in a time-averaged sense to some extent, there are still unanswered questions at a fundamental level about transient dispersion process and thermal boundary conditions, calling for further studies with more advanced models for simulations and more sound experiments for validations.

Air exchange rates from atmospheric CO2 daily cycle

We propose a new approach for measuring ventilation air exchange rates (AERs). The method belongs to the class of tracer gas techniques, but is formulated in the light of systems theory and signal processing. Unlike conventional CO₂ based methods that assume the outdoor ambient CO₂ concentration is constant, the proposed method recognizes that photosynthesis and respiration cycle of plants and processes associated with fuel combustion produce daily, quasi-periodic, variations in the ambient CO₂ concentrations. These daily variations, which are within the detection range of existing monitoring equipment, are utilized for estimating ventilation rates without the need of a source of CO₂ in the building. Using a naturally-ventilated residential apartment, AERs obtained using the new method compared favorably (within 10%) to those obtained using the conventional CO₂ decay fitting technique. The new method has the advantages that no tracer gas injection is needed, and high time resolution results are obtained.

CO2 generation rate in Chinese people

Carbon dioxide (CO2 ) metabolically produced by humans has been widely used as a tracer gas for determining ventilation rates in occupied rooms. Among other necessities, the method requires good estimates of human CO2 generation rates. An empirically derived equation is widely used to calculate the CO2 generation rate. However, there are indications that this equation is not valid for young Chinese people. In this study, we measured the CO2 generation rate of 44 young Chinese people at two typical activity levels, quiet sitting and relaxed standing. We found that the commonly used empirical equation overpredicted CO2 generation rates, but could be corrected with a factor of 0.75 for Chinese females and of 0.85 for Chinese males. The variance for measured CO2 sitting was much smaller than for standing, and hence, we concluded that sitting yields more precise CO2 generation estimates. The relative contributions of sex, height, weight, and metabolic rate were analyzed. We concluded that the error in estimating metabolic rate is responsible for most of the difference in measured generation of CO2 from the empirical equation's predictions.

PRACTICAL IMPLICATIONS

The tracer gas method using CO2 generated by people is widely used to calculate ventilation rate. However, the empirically derived equation that is normally used to estimate CO2 generation rate is not suitable for young Chinese people at rest. To estimate the CO2 generation rate in Chinese people under low-activity conditions, the empirical equation should be multiplied by correction factors of 0.75 and 0.85 for females and males, respectively.

Numerical investigation of wind-induced airflow and interunit dispersion characteristics in multistory residential buildings

Compared with the buoyancy-dominated upward spread, the interunit dispersion of pollutants in wind-dominated conditions is expected to be more complex and multiple. The aim of this study is to investigate the wind-induced airflow and interunit pollutant dispersion in typical multistory residential buildings using computational fluid dynamics. The mathematical model used is the nonstandard k-ε model incorporated with a two-layer near-wall modification, which is validated against experiments of previous investigators. Using tracer gas technique, the reentry of exhaust air from each distinct unit to other units on the same building, under different practical conditions, is quantified, and then, the possible dispersion routes are revealed. The units on the floor immediately below the source on the windward side, and vertically above it on the leeward side, where the reentry ratios are up to 4.8% and 14.9%, respectively, should be included on the high-infection list. It is also found that the presence of balconies results in a more turbulent near-wall flow field, which in turn significantly changes the reentry characteristics. Comparison of the dispersion characteristics of the slab-like building and the more complicated building in cross (#) floorplan concludes that distinctive infectious control measures should be implemented in these two types of buildings.

Numerical investigation of wind-induced airflow and interunit dispersion characteristics in multistory residential buildings

Compared with the buoyancy-dominated upward spread, the interunit dispersion of pollutants in wind-dominated conditions is expected to be more complex and multiple. The aim of this study is to investigate the wind-induced airflow and interunit pollutant dispersion in typical multistory residential buildings using computational fluid dynamics. The mathematical model used is the nonstandard k-ε model incorporated with a two-layer near-wall modification, which is validated against experiments of previous investigators. Using tracer gas technique, the reentry of exhaust air from each distinct unit to other units on the same building, under different practical conditions, is quantified, and then, the possible dispersion routes are revealed. The units on the floor immediately below the source on the windward side, and vertically above it on the leeward side, where the reentry ratios are up to 4.8% and 14.9%, respectively, should be included on the high-infection list. It is also found that the presence of balconies results in a more turbulent near-wall flow field, which in turn significantly changes the reentry characteristics. Comparison of the dispersion characteristics of the slab-like building and the more complicated building in cross (#) floorplan concludes that distinctive infectious control measures should be implemented in these two types of buildings.

The airborne transmission of infection between flats in high-rise residential buildings: Tracer gas simulation

Airborne transmission of infectious respiratory diseases in indoor environments has drawn our attention for decades, and this issue is revitalized with the outbreak of severe acute respiratory syndrome (SARS). One of the concerns is that there may be multiple transmission routes across households in high-rise residential buildings, one of which is the natural ventilative airflow through open windows between flats, caused by buoyancy effects. Our early on-site measurement using tracer gases confirmed qualitatively and quantitatively that the re-entry of the exhaust-polluted air from the window of the lower floor into the adjacent upper floor is a fact. This study presents the modeling of this cascade effect using computational fluid dynamics (CFD) technique. It is found that the presence of the pollutants generated in the lower floor is generally lower in the immediate upper floor by two orders of magnitude, but the risk of infection calculated by the Wells-Riley equation is only around one order of magnitude lower. It is found that, with single-side open-window conditions, wind blowing perpendicularly to the building may either reinforce or suppress the upward transport, depending on the wind speed. High-speed winds can restrain the convective transfer of heat and mass between flats, functioning like an air curtain. Despite the complexities of the air flow involved, it is clear that this transmission route should be taken into account in infection control.