Layered COVID-19 controls significantly reduce risks

QuaRAD方法在数千种场景中量化疗效;rating system anticipated

By Kevin Parker April 8, 2022
由Daniel Penn Associates提供。

Use of a hierarchy of controls framework can mitigate COVID-19 transmission risk by identifying best practices and shared indoor-air quality standards. The idea has been studied by Naples, FL-based Daniel Penn Associates, working with Brookhaven National Laboratory under a U.S. Department of Energy grant.

“While masking and distancing are important elements in reducing virus transmission, our study results place equal emphasis on the need for updating and upgrading ventilation, filtration and removal systems,” said Tony Rodriguez, president of Daniel Penn Associates. “The quality of shared air indoors is affected by these pathways. The ideal method is physical removal of the hazard. Least effective is use of PPE [personal protective equipment].”

The study examined implementation and effectiveness of Covid-19 controls, including face masks, ventilation and physical distancing. While efficacy of any individual control proved highly variable, the study found combining universal mask wearing with distancing of one meter or more reduced median exposure by more than 99% relative to a close, unmasked conversation, with further reduction given enhanced ventilation settings.

The report’s principal investigators were Brookhaven’s Dr. Robert McGraw and Dr. Jason Farrell, president of Facilitiesmart.

Dr. McGraw leads the Aerosol Chemistry and Micropysics Group within the Department of Environment and Climate Science at Brookhaven and is the originator of the highly efficient quadrature method of moments developed under DOE and NASA funding for use in atmospheric aerosol and climate models.

Facilitiesmart is a consulting firm focused on guiding, organizing and orchestrating effective human configurations of spaces and facilities.

Back story of controls

虽然人们普遍认为COVID-19的传播主要通过空气传播,但缺乏关于如何建立有效控制系统以缓解封闭空间传播的指导。

Controls are defined by the U.S. Occupational Safety and Health Association (OSHA) as the use of engineering methods to reduce hazard levels inside confined spaces. The Hierarchy of Controls is a framework through which a system of controls can be examined.

Implemented correctly, common controls for airborne transmission of COVID-19 reduce the number of airborne virus-laden particles inhaled by a susceptible person. However, how well they work depends upon factors inherently variable and poorly constrained. For example, while the efficiency of face masks is known, reducing infection risk also depends on viral shedding, characteristics of expiratory jets, room conditions — such as rates of air exchange and relative humidity — and immune responses.

Besides reviewing selected research, the report blends expertise in engineering controls and sociological behavior (Daniel Penn and Facilitiesmart) with expertise in atmospheric aerosol chemical and microphysical processes and transport (Brookhaven). Daniel Penn Associates was the only private-sector company approved for a DoE grant to conduct this research review.

Science of controls

使用OSHA的控制层次作为透镜,该报告明确了当前对控制实施和有效性的理解。“一套科学支持的参数将允许对物理设施进行‘塑造’,以帮助关闭大流行暴露的主要脆弱性——共享室内空气。目前,室内空气空间的管理主要集中在热舒适和能效方面。在大多数人的意识中,共享室内空间的个体之间的病原体传播并不像共享食物或水那样。”法雷尔说。

As a respiratory virus, COVID-19 infection occurs when virus-laden particles are inhaled by an individual. This limits the effectiveness of control measures such as high-touch surface sanitization, physical-contact avoidance, and hand washing.

Hazardous work characteristics that do contribute to worker vulnerability include exposure to infected aerosols; lack of effective PPE; prolonged near-field exposures; and densely populated, enclosed or poorly ventilated spaces.

The Hierarchy of Controls is a graphical display of desired effects of any control method, with those that are more protective at the top of the graphic. Each category’s size serves as a visual representation of the strength and potential ability to reduce the risk of injury or illness.

“OSHA developed the Hierarchy of Controls to provide guidance around the most effective ways to mitigate the risk of workplace hazards, and to provide a lens through which an organization’s safety practices might be evaluated,” Farrell said.

Unfortunately, one risk is that employers can face temptations to take short cuts with worker safety.

“The model, when applied, can represent the vulnerability of regular citizens as they engage with shared societal spaces. It can provide similar piece of mind to employees in settings regulated by a standard set of safety parameters,” Farrell said.

Masks can reduce virus-laden particle emissions exhaled into surrounding air and filter those same particles from inhaled air. However, filtration efficiency of the various types, materials, and fits varies dramatically, making it difficult to predict the safety of a space unless mask usage is fully controlled.

Proper distancing provides relief from the risk posed in the near-field expiratory jet of infected individuals but does little to control far-field infections. An effective layered control system requires a blended approach that incorporates multiple methods.

QuaRAD as a control model

Uncertainty in measuring control efficacy can be modelled by simulating ensembles of scenarios. However, models well-suited for simulating the evolution of respiratory particles in indoor spaces are often computationally expensive, limiting their ability to represent uncertainty. Simulations using the quadrature Based Model of Respiratory Aerosol and Droplets (QuaRAD) overcome this challenge.

QuaRAD applies the Quadrature Method of Moments (QMOM) to aerosol dynamics simulation. QuaRAD is part of the tremendous advances made in recent decades in advanced statistical methods that quantify the efficacy of layered controls.

“The method replaces the full particle distribution function with just a few quadrature points. For the simulation of near-field enhancement in transmission of airborne viruses, three points were found to be too few and six gave convergence, as verified against Monte Carlo simulations involving thousands of particles,” said McGraw.

QuaRAD adapts the QMOM method to simulate the evolution of virus-laden particles expelled from an infectious person. Ten thousand representative scenarios were simulated to quantify the risk of initial infection by COVID-19.

“在合理的计算机时间内,我们对数千个真实的场景进行了采样、模拟和分析,这些场景代表了各种各样的室内空气条件和一般人群的生理特性。通过跟踪排出的颗粒的大小分布,以及它们在蒸发、运输和去除过程中的演变,模拟量化了面部覆盖物、通风和距离的单独和综合效果。”McGraw说。

In fact, the simulations helped refine understanding of the influence of multiple variables and specific parameters.

Detailed findings

与近距离、不戴口罩的交谈相比,通用口罩佩戴和1米及以上的距离可以减少99%以上的中位暴露,如果还加强通风,则会进一步减少(图3)。这表明,分层控制可以有效减少空气传播病原体,并将对遏制未来新病毒的爆发至关重要。

控制层次结构:此图是为工业环境或工作环境开发的,这里用作空气传播流行病情况的模型。

Fig. 1: Hierarchy of Controls: This figure, developed for an industrial setting or work environment, is used here as a model for airborne pandemic situations. Courtesy: Daniel Penn Associates.

Amongst the epidemiological puzzles of the SARS-CoV-19 pandemic are surprisingly large heterogeneities in individual response to infection and large fluctuations in the rate of transmission. These fluctuations are greatly amplified by heterogeneity in immune response from prior exposure and vaccination state. Most dose-response functions in current use stop with a determination of the probability of infection, as was done here with QuaRAD.

This approach neglects the heterogeneity of individual immune response, the probability of the host becoming ill, and the propensity of that host infecting others once infected. Particle population balance models, like those used in aerosol science, should be easy to modify to consider the competition between virion multiplication and heterogeneity of immune response

该研究的参与者认为,开发一个基于科学的系统来量化特定室内环境与大流行相关的安全性,并在窗户、门或网站上显示相应的评级,将为雇主和雇员建立信心。

“A focus on the quality of shared air indoors has the great potential to reduce vulnerability and increase confidence as we attempt to return to normal conditions,” Farrell said.


Kevin Parker



Author Bio:Senior contributing editor, CFE Media