As counties in Pennsylvania and other states begin a phased reopening plan, many people are concerned about the continued spread of COVID-19 or another spike in cases. While health officials stress the importance of handwashing and wearing masks, we want to share highlights of an article that can guide you away from situations of high risk.
COVID-19: The Risks – Know Them – Avoid Them
Published on May 6, 2020 by Erin S. Bromage, Ph.D.
Where are people getting sick?
We know most people get infected in their own home. A household member contracts the virus in the community and brings it into the house where sustained contact between household members leads to infection.
In order to get infected, you need to get exposed to an infectious dose of the virus; based on infectious dose studies with other coronaviruses, it appears that only small doses may be needed for infection to take hold. Some experts estimate that as few as 1000 SARS-CoV2 infectious viral particles are all that will be needed. Please note, this still needs to be determined experimentally, but we can use that number to demonstrate how infection can occur. Infection could occur, through 1000 infectious viral particles you receive in one breath or from one eye-rub, or 100 viral particles inhaled with each breath over 10 breaths, or 10 viral particles with 100 breaths. Each of these situations can lead to an infection.
How much Virus is released into the environment?
A Bathroom: Bathrooms have a lot of high touch surfaces, door handles, faucets, stall doors and the transfer risk in this environment can be high. We still do not know whether a person releases infectious material in feces or just fragmented virus, but we do know that toilet flushing does aerosolize many droplets. Treat public bathrooms with extra caution (surface and air), until we know more about the risk.
A Cough: A single cough releases about 3,000 droplets and droplets travels at 50 miles per hour. Most droplets are large, and fall quickly (gravity), but many do stay in the air and can travel across a room in a few seconds.
A Sneeze: A single sneeze releases about 30,000 droplets, with droplets traveling at up to 200 miles per hour. Most droplets are small and travel great distances (easily across a room). If a person is infected, the droplets in a single cough or sneeze may contain as many as 200,000,000 (two hundred million) virus particles which can all be dispersed into the environment around them.
A Breath: A single breath releases 50 – 5000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released through nose-breathing. Importantly, due to the lack of exhalation force with a breath, viral particles from the lower respiratory areas are not expelled. Unlike sneezing and coughing which release huge amounts of viral material, the respiratory droplets released from breathing only contain low levels of virus.
Remember the formula: Successful Infection = Exposure to Virus x Time
- If a person coughs or sneezes, the viral particles go everywhere. If you are face-to-face with a person, having a conversation, and that person sneezes or coughs straight at you, it is possible to inhale 1,000 virus particles and become infected. Even if that cough or sneeze was not directed at you, infectious viral particles can fill every corner of a modest sized room. All you have to do is enter that room within a few minutes, take a few breaths and you have potentially received enough virus to establish an infection.
- With general breathing of 20 viral particles per minute into the environment, even if every virus ended up in your lungs (which is very unlikely), you would need 1000 viral particles divided by 20 per minute = 50 minutes.
- Speaking increases the release of respiratory droplets about 10-fold. Assuming every virus is inhaled, it would take 5 minutes of speaking face-to-face to receive the required dose.
- Anyone you spend greater than 10 minutes with in a face-to-face situation is potentially infected. Anyone who shares a space with you (say an office) for an extended period is potentially infected.
- It is critical for people who are symptomatic to stay home. Your sneezes and your coughs expel so much virus that you can infect a whole room of people.
What is the role of asymptomatic people in spreading the virus?
Symptomatic people are not the only way the virus is shed. We know that at least 44% of all infections occur from people without any symptoms (asymptomatic or pre-symptomatic people). You can be shedding the virus into the environment for up to 5 days before symptoms begin.
Infectious people come in all ages, and they all shed different amounts of virus. The amount of virus released from an infected person changes over the course of infection and it is also different from person-to-person. Viral load generally builds up to the point where the person becomes symptomatic. Just prior to symptoms showing, you are releasing the most virus into the environment.
Ignoring the terrible outbreaks in nursing homes, we find that the biggest outbreaks are in prisons, religious ceremonies, and workplaces, such as meat packing facilities and call centers. Any environment that is enclosed, with poor air circulation and high density of people, spells trouble.
As we move back to work, or go to a restaurant, let’s look at what can happen in those environments:
- Restaurants: A single asymptomatic carrier releases low-levels of virus into the air from their breathing. Because of the airflow from vents, approximately 50% of the people at the infected person’s table became sick, 75% of the people on the adjacent downwind table became infected and a few of the people on the upwind table were infected (believed to happen by turbulent airflow). No one out of the main airflow from the air conditioner became infected.
- Call Center: A single infected employee infected 43.5% of the workplace (mostly one side of the room). While the exact number of people infected by respiratory droplets / respiratory exposure versus surface transmission is unknown, it serves to highlight that being in an enclosed space, sharing the same air for a prolonged period increases your chances of exposure and infection.
- Meat Processing Plant: Densely packed workers must communicate to one another amidst the deafening drum of industrial machinery and a cold-room virus-preserving environment.
- Business Networking: Face-to-face business networking
- Choir: A single asymptomatic carrier infected most of the people in attendance after singing inside for 2.5 hours – even though people took steps to minimize transfer (ex. no hugging or handshakes). Singing, to a greater degree than talking, aerosolizes respiratory droplets extraordinarily well. Deep-breathing while singing facilitated those respiratory droplets getting deep into the lungs.
- Indoor sports: A sporting event brings athletes and teammates in close contact in a cool indoor environment, with heavy breathing for an extended period.
- Birthday parties / funerals: An infected person shares a takeout meal served from common dishes and then attends a funeral, hugging family members and others in attendance to express condolences the next day. He also attended a birthday party where he hugged and shared food with other people. The spread of the virus within the household and back out into the community through funerals, birthdays, and church gatherings is believed to be responsible for the broad transmission of COVID-19.
Commonality of Outbreaks
The reason to highlight these different outbreaks is to show you the commonality of outbreaks of COVID-19. All these infection events were indoors, with people closely-spaced, with lots of talking, singing, or yelling. The main sources for infection are home, workplace, public transport, social gatherings, and restaurants. This accounts for 90% of all transmission events. In contrast, outbreaks spread from shopping appear to be responsible for a small percentage of traced infections.
Indoor spaces, with limited air exchange or recycled air and lots of people, are concerning from a transmission standpoint. Social distancing guidelines don’t hold in indoor spaces where you spend a lot of time. The principle is viral exposure over an extended period of time.
Social distancing rules are really to protect you with brief exposures or outdoor exposures. In these situations, there is not enough time to achieve the infectious viral load when you are standing 6 feet apart or where wind and the infinite outdoor space for viral dilution reduces viral load. The effects of sunlight, heat, and humidity on viral survival, all serve to minimize the risk to everyone when outside.
When assessing the risk of infection (via respiration) at the grocery store or mall, you need to consider the volume of the air space (very large), the number of people (restricted), how long people are spending in the store (workers – all day; customers – an hour). Taken together, for a person shopping: the low density, high air volume of the store, along with the restricted time you spend in the store, means that the opportunity to receive an infectious dose is low. But, for the store worker, the extended time they spend in the store provides a greater opportunity to receive the infectious dose and therefore the job becomes riskier.
As the work closures are loosened, and we start to venture out more, you need to look at your environment, make judgments and assess the risk:
- How many people are here?
- How much airflow is there around me?
- How long will I be in this environment?
- Am I in an open floorplan office? What’s the of volume of people and airflow?
- Does my job require face-to-face talking or yelling?
- Am I sitting in a well-ventilated space with few people? (low risk)
If you are outside, and walk past someone, remember it is “dose and time” needed for infection. You would have to be in their airstream for 5+ minutes for a chance of infection. While joggers may be releasing more virus due to deep breathing, remember the exposure time is also less due to their speed. Please do maintain physical distance, but the risk of infection in these scenarios are low.
While this article focused on respiratory exposure here, please don’t forget surfaces. Those infected respiratory droplets land somewhere. Wash your hands often and stop touching your face!
As we are allowed to move around our communities more freely and be in contact with more people in more places more regularly, the risks to ourselves and our family are significant. Do your part and wear a mask to reduce what you release into the environment. It will help everyone.
To read the full article and see graphs associated with the author’s research, please visit https://www.erinbromage.com/post/the-risks-know-them-avoid-them