Dr. William Ristenpart.
As you may have heard, Marin County Public Health Officer Dr. Matt Willis indicated this week that Marin’s COVID-19 metrics continue to improve to the point that we could advance to the “orange” tier 2 within the statewide Blueprint for a Safer Economy framework as soon as Oct. 27, a move that would allow several business sectors, including restaurants, to move from 25% to 50% density indoors.
Many business owners have said that getting to 50% density was the minimum they’d need to be able to create a business model that might pencil out, e.g., takeout/delivery + some outdoor dining + some indoor dining for restaurants.
So in the (fingers-crossed) hope that increased density guidelines improve later this month, we wanted to help you take the necessary steps to improve your restaurant, cafe or retail space and protect your employees and customers by maximizing the amount of outside air being delivered to your space.
To do so, Mill Valley Lumber Yard’s Matt and Jan Mathews and Equator Coffees’ Helen Russell reached out to Dr. William Ristenpart, a professor of chemical engineering at UC Davis and director of the university’s Coffee Center for a tutorial on the science of airborne disease transmission and how to reduce airborne disease transmission in your business.
The nearly two-hour presentation is embedded in full below and chock full of excellent detail about how airborne diseases like coronavirus are transmitted and what you can do to improve the safety of your space. But in our continued mindfulness of your time, we’ve distilled the presentation below:
Early in the COVID-19 era, scientists focused on transmission via contact (handshakes) and fomite (door handles). But now there is a lot of evidence pointing to “aerosol” – droplets or simply breath – as a major form of transmission, says Ristenpart, with people getting infected in spaces in which they had no direct contact with an infected person. Also, studies have shown that as many as 86% of transmissions come from people with “mild, limited or no symptoms.”
Despite repeated mixed signals from the Centers for Disease Control and Prevention in recent weeks, a group of aerosol scientists reported that "there is overwhelming evidence that inhalation represents a major transmission route.”
“Plume” vs “Room”
Respiratory plumes, or particle emissions from people who are simply breathing and talking, particularly in close proximity, can be potent forms of transmission, as speech emits a large amount of particles, and the louder you are, the more particles that come out.
“This can potentially be the biggest source of transmission from asymptomatic people,” Ristenpart says. Room transmission is caused by long distance aerosol spread through a room. Though the transmission occurs in lower concentrations, it can be a significant factor depending on the air flow, and the amount of fresh air circulation within a space.
“All the people that were infected at the top part were in the middle of this recirculatory air conditioning flow,” Ristenpart says of the graphic below. “And the longer you’re exposed to the pathogens the more you are likely to inhale.”
Many business owners have said that getting to 50% density was the minimum they’d need to be able to create a business model that might pencil out, e.g., takeout/delivery + some outdoor dining + some indoor dining for restaurants.
So in the (fingers-crossed) hope that increased density guidelines improve later this month, we wanted to help you take the necessary steps to improve your restaurant, cafe or retail space and protect your employees and customers by maximizing the amount of outside air being delivered to your space.
To do so, Mill Valley Lumber Yard’s Matt and Jan Mathews and Equator Coffees’ Helen Russell reached out to Dr. William Ristenpart, a professor of chemical engineering at UC Davis and director of the university’s Coffee Center for a tutorial on the science of airborne disease transmission and how to reduce airborne disease transmission in your business.
The nearly two-hour presentation is embedded in full below and chock full of excellent detail about how airborne diseases like coronavirus are transmitted and what you can do to improve the safety of your space. But in our continued mindfulness of your time, we’ve distilled the presentation below:
Early in the COVID-19 era, scientists focused on transmission via contact (handshakes) and fomite (door handles). But now there is a lot of evidence pointing to “aerosol” – droplets or simply breath – as a major form of transmission, says Ristenpart, with people getting infected in spaces in which they had no direct contact with an infected person. Also, studies have shown that as many as 86% of transmissions come from people with “mild, limited or no symptoms.”
Despite repeated mixed signals from the Centers for Disease Control and Prevention in recent weeks, a group of aerosol scientists reported that "there is overwhelming evidence that inhalation represents a major transmission route.”
“Plume” vs “Room”
Respiratory plumes, or particle emissions from people who are simply breathing and talking, particularly in close proximity, can be potent forms of transmission, as speech emits a large amount of particles, and the louder you are, the more particles that come out.
“This can potentially be the biggest source of transmission from asymptomatic people,” Ristenpart says. Room transmission is caused by long distance aerosol spread through a room. Though the transmission occurs in lower concentrations, it can be a significant factor depending on the air flow, and the amount of fresh air circulation within a space.
“All the people that were infected at the top part were in the middle of this recirculatory air conditioning flow,” Ristenpart says of the graphic below. “And the longer you’re exposed to the pathogens the more you are likely to inhale.”
A graphic of an air conditioning unit recirculating air without infusing the room with fresh air from outside.
Size Up Your Space
Determine “Room Volume”: The starting point for you to figure out how to improve the air flow within your space is length x width x height or, more simply, square footage x height.
Determine “Ventilation Flow Rate”: This crucial measurement of the amount of air change per hour is critically important, Ristenpart says. HVAC units supply air from the central air handler and recirculates it with fresh air, and it is highly recommended that those buildings with HVAC units contact an HVAC technician to help them through the process of determining how many cubic feet per hour of fresh air are you delivering to your space, and what must be done to improve it.
CDC guidelines call for small restaurants, for instance, to have four air changes per hour, depending on the size of the space, while the National Comfort Institute calls for 8-10 air flow changes per hour in dining areas.
Here are recommendations to make your space as safe as possible for you, your customers and your employees:
Determine “Ventilation Flow Rate”: This crucial measurement of the amount of air change per hour is critically important, Ristenpart says. HVAC units supply air from the central air handler and recirculates it with fresh air, and it is highly recommended that those buildings with HVAC units contact an HVAC technician to help them through the process of determining how many cubic feet per hour of fresh air are you delivering to your space, and what must be done to improve it.
CDC guidelines call for small restaurants, for instance, to have four air changes per hour, depending on the size of the space, while the National Comfort Institute calls for 8-10 air flow changes per hour in dining areas.
Here are recommendations to make your space as safe as possible for you, your customers and your employees:
- Be wary of your use of conditioners: Comfort is obviously important, Ristenpart notes, and central air conditioners typically deliver fresh air and/or filters the recirculated air (assuming you have inline filtration). But he cautions against ductless AC air conditioning unless there is an introduction of outdoor air and/or additional HEPA filtering used in conjunction with it.
- Masks are mandatory: Require customers to keep masks on at all times except when they are actively sipping their coffee and eating food. Face shields may block plume transmission but not room transmission, so mask usage is critical. The same is true with plastic partitions, as plumes can simply travel around partitions. Partitions may be useful if deployed in very high interaction locations.
- Increase your ventilation: Find out what the “air changes per hour” is in your space and if you need to improve it. “By far the best ventilation is being outside, as the air changes per hour is essentially infinite,” Ristenpart says. “And open the windows as much as possible. Use window fans or freestanding fans to help direct air from outside.
- If your building has an HVAC unit, get advice from a technician on how best to maximize the amount of outside air being delivered to your space – recirculation is not ventilation, so it won't be effective to turn on a ceiling fan. Ristenpart suggested what he called a crude but clear analogy: “That would be like peeing in a bathtub and saying ‘I’m going to clean it up by stirring it.’”
- Get an air purifier: “They are fantastic,” Ristenpart says. When buying a purifier, focus on the MERV rating, or the “minimum efficiency reporting value.” The higher the rating, the more small particles it catches. ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, recommends a MERV rating of at least 13-14.
- Locate as much of your air purification and fresh air inflow efforts in areas where employees and customers cluster most frequently.
- Limit your exposure: Encourage brief and quiet use of your space. Perhaps set time limits on reservations or appointments. Encourage outdoor use after a sales transaction. If your space has music playing, keep the volume down to discourage loud conversations.
- Encourage employees to stay at home if they’re feeling sick and get rid of policies that encourage policies to work despite illness.