Dr. Gavin Macgregor-Skinner

Buildings and indoor spaces have been associated with the spread of infectious diseases through the air, such as measles, influenza, tuberculosis, Legionnaires’ disease, and COVID-19. People are often the source of infection, and your chances of being infected depend on the size of the room and the number of people in it. Just as you might think of a room full of smokers and the smoke they generate, you can also think of people with a contagious illness releasing viruses and bacteria into the air. According to the U.S. Centers for Disease Control and Prevention (CDC), infectious diseases can spread when an infected person coughs, sneezes, talks, or breathes out droplets and tiny particles that contain viruses or bacteria. These droplets and particles can be breathed in by other people or land on their eyes, noses, or mouth. An infected person can contaminate surfaces they touch. Virus particles and bacteria in the air can settle on and contaminate surfaces. People can get infected by touching their eyes, nose, or mouth with hands contaminated by a virus or bacteria. A study from National Institutes of Health (NIH); CDC; University of California, Los Angeles; and Princeton University scientists in The New England Journal of Medicine found that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes COVID-19 was detectable in respiratory droplets in the air for up to three hours. A single cough can expel more bacteria or viruses than a single breath. But if an infected person breathes 22,000 times per day while coughing up to 500 times, then coughing accounts for as little as 7% of the total bacteria or virus emitted by an infected patient. What is ventilation rate and air change rate? Knowing air quality terminology can help you ask the right questions when entering any indoor space. Ventilation rate is the volume of outside air provided per unit of time. Air change rate is the ventilation rate of a space divided by the volume of that space. The rate that outdoor air replaces indoor air. ASHRAE, formerly called the American Society of Heating, Refrigerating and Air-Conditioning Engineers, provides outside air ventilation standards for commercial buildings, including schools, hotels, convention centers, and airports. These minimum ventilation rates vary depending on the type of activity occurring in the room. Outdoor air dilutes contaminants produced by the occupants and the building itself, so rates are based on both the maximum number of people the room is designed to hold and the size of the room. Ventilation air only benefits occupants if it reaches them. Therefore, we should consider the minimum ventilation rate within the “breathing zone.” ASHRAE defines the breathing zone as the region within an occupied space between three and six feet above the floor and more than two feet from walls or fixed air-conditioning equipment. How much ventilation do I need? ANSI/ASHRAE Standards 62.1 and 62.2 are the recognized standards for ventilation system design and acceptable indoor air quality (IAQ). Standard 62.1 Ventilation for Acceptable Indoor Air Quality outlines minimum ventilation rates based on the combination of cubic feet per minute (cfm) per person and per square foot of floor area, and both are related to the occupancy type. It includes three procedures for ventilation design (the Indoor Air Quality Procedure, the Ventilation Rate Procedure, and the Natural Ventilation Procedure) intended to provide indoor air quality that is acceptable to human occupants and that minimize adverse health effects. For example, museums/galleries require a minimum combined outdoor air rate of nine cfm/person, while animal areas in pet shops or weight rooms in health clubs require no less than 26 cfm/person. Office spaces require 17 cfm/person; however, an auditorium seating area only requires five cfm/person. Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Residential Buildings recommends that homes receive 0.35 air changes per hour but not less than 15 cubic feet of air per minute per person, as the minimum ventilation rates in residential buildings to provide indoor air quality that is acceptable to human occupants and that minimizes adverse health effects. With respect to engineering controls, an important flaw exists in that the current standards for ventilation are set for bare minimums and not designed for infection control. The recommended ventilation rate before the pandemic For a 1,000-square-foot classroom designed for 35 people, including teachers and students ages nine and older, ASHRAE recommends a ventilation rate of 500 cfm of outside air. This means there are 13 cubic feet per minute per person. If the ceiling height in the classroom is 10 feet, then the room needs three total outside air changes per hour. To calculate the number of air changes per hour, take the ventilation rate in cfm and multiply by 60 minutes and divide by the room volume (length x width x height): (500 cfm x 60 minutes) / (10,000 ft3) = three air changes per hour. What does reduced occupancy do to improve air quality? One way to reduce the risk of disease transmission is to reduce the number of people in a room. This allows for social distancing that reduces risk of close contact transmission. It also reduces the number of probable infections that will occur if there is an infected person in the room. And as an added benefit, more outside air is provided per person, contributing to better overall air quality. If we take the same classroom of 1,000 square feet and reduce the number of people from 35 to 17 by cutting the occupancy in half, the ventilation rate now provides twice as much outside air per person. In the 1,000-square-foot classroom with a 10-foot ceiling and a ventilation rate of 500 cfm of outside air, for 17 people ages nine and older, we now have 26 cfm/per person of outside air, and by still maintaining three air changes per hour. Increasing ventilation with outside air has been proven to reduce the transmission of airborne diseases by lowering the concentration of infectious particles in the air. The reduced class size now allows space for social distancing. Wearing a

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Dr. Gavin Macgregor-Skinner

Senior Director, GBAC

Articles by Dr. Gavin Macgregor-Skinner

Indoor Air Quality + Air Purification

Infection Control

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