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# How to know if it’s safe outside

We are deluged with so many different COVID19 numbers. Epidemiologists care about one set of numbers, public health administrators another set, and hospitals and healthcare providers a different set of numbers. The politicians try to put this together in a way that helps the public without risking their political careers. Government bureaucrats give us data that is both politically correct and heavily flavored with CYA. But if you’re just a simple citizen, like me, the only question you have is “what are my chances of crossing paths with a person spreading the virus?” Let’s call that the Exposure Number. Give us the Exposure Number and each person can decide if they want to take that chance.

We already know so much. We know that many people spread the virus without being sick. We know that most people who get sick do fine after a few days, but some stay sick a bit longer. Some get really sick and have to go to the hospital. A few of those people end up on ventilators and sadly some pass. We know who is at high risk for a bad outcome and who isn’t. We also know that many of the cases are in nursing homes. We are smart enough to digest this information and realize that there are always a few exceptions to every rule. Once in a while a young healthy person has a bad outcome and a very old person recovers. The newsworthiness of these exceptions proves the rule.

Before I show you how easy it is to calculate the Exposure Number, or the chance of crossing paths with a random person spreading the virus, ask yourself what you’ll do with this number. That’s the hard question. Have you ever gauged your level of risk aversion? Probably not. Is 1 in a 100 rare enough? How about 1 in 500 or 1 in 1000? Do you only feel safe at 1 in 10,000? Most people don’t think statistically and have never even asked themselves that question. You know somebody will get killed on the road today but you don’t do that analysis. You just assume it won’t be you and get in the car. One needs context to answer a risk question. But let’s start by calculating the Exposure Number. What are the chances you will cross paths with someone carrying the virus? To answer this question, we only need 2 numbers:

• How many new cases are diagnosed each day?
• How many of those cases are in nursing homes? Nursing home patients won’t cross our path.

We also need to make a few assumptions.

• First, let’s assume that every person who has the virus and isn’t in a nursing home is walking about freely. Even though many aren’t because they feel ill and stay home, I want to be conservative.
• Second, let’s assume that the average person with the virus will spread it for 10 days. Some spread it longer and some shorter. The average is less than this but I want to be conservative.
• Third, let’s assume that for every person who had symptoms and went to get tested, there are 2 additional people who are positive but either didn’t have symptoms or didn’t feel like getting tested.

For example, if 100 new people tested positive today and every day for the last 10 days, and it takes 10 days to stop spreading virus, there will be 1000 people walking around today spreading virus. Divide that by the number of people in the group being tested and that tells you your chance of crossing paths with a viral spreader.

As applied to Miami, where I live, on April 24th there were 383 new positives. My assumptions tell us there an additional 766 untested positives, for a total of 1149 new active spreaders in Miami on April 24th. Over the last 10 days, the data and my assumptions show the total to be 26,766 people in Miami spreading virus. Since 26% of all our positives are in nursing homes, that means that there are 20,342 people in Miami spreading virus. Since the population of Miami is 2,717,000, that means that 1 out of every 303 people in Miami is spreading virus. In Broward County, the number is 1 on 482. For context, about 1 in 550 people in the US are on dialysis. How often do you come across a dialysis patient?

Now comes the hard part. Can you tolerate 1 spreader in 303 or 482 random people? To help answer that question, let’s bring in some facts. The CDC claims that simply walking by or very briefly standing in the vicinity of a positive patient isn’t even a low risk exposure. It’s possible you’ll get it from a positive person in that situation, but only remotely probable so we don’t count it. That’s what statistical thinking is about. Even a low risk exposure requires a few minutes of close contact, such as sitting within a few feet of a spreader for a few minutes. Moreover, the virus isn’t evenly distributed in the community. Just like 24% of the positives are in nursing homes, many of the other positives are likely to live in high density areas, such as housing projects or crowded neighborhoods, or gather in crowds for cultural reasons. Population density is a major factor in spread. So, if you and your frequent contacts don’t live in high density areas and haven’t been crowding together, your odds got a whole lot better. Outdoor activities in sunlight make transmission even less likely. Even if you spend some time with a spreader, there is only about a 1 in 6 chance you’ll get it, based on the findings on the cruise ship and aircraft carrier. Even if you get it, you might have no symptoms or only mild symptoms. Yes, some young healthy people do die but those are rare enough that it makes the news. Sort of like “man bites dog” news stories. Nonetheless, the elderly and people with underlying medical conditions should be more careful.

Ask yourself how many random people a day do you spend time with and how much time do you spend with them? If you are meeting and greeting dozens of new people every day, your likelihood of running into a random spreader is much greater than if your circle is more limited. Each person will have to decide the Exposure Number at which they feel safe and determine how close they are to reaching that number.

My recommendation is that in areas where the hospital systems haven’t been overwhelmed, which is in most of world, the authorities should give us the Exposure Number and let us make our own decisions. Each person, in the context of their age, needs, lifestyle and health status can decide if he wants to go out or stay indoors, if and where they want to wear a mask, and to whom they will allow themselves to be exposed. Making an assessment based on the Exposure Number, in conjunction with hand washing as needed, social distancing from random people we don’t know to be low risk and the application of some common sense will allow many of us to get back to some semblance of normal life sooner than later.