With the spread of novel coronavirus (COVID-19) across the globe many governments and organizations have taken to recommending or requiring people to stay indoors (1, 2). However given what we know about the flu and previous coronaviruses, this advice may in fact be counterproductive.
Influenza and previous coronaviruses (which make up one-third of common cold viruses) show a 'seasonal' pattern. In temperate climates they spike in winter and fade in summer. The commonly-accepted reason is that in winter people tend to stay indoors where temperatures and humidity levels are moderate (approximately 72° and 50% respectively). Under such conditions viruses can survive on smooth surfaces for up to 5 days (3). Viruses also tend to stay airborne longer under colder and dryer conditions than they do under warmer and more humid conditions (4).
During summer people spend much more time outside where the temperature and relative humidity are often significantly higher (greater than 85° and/or 70% respectively). Plus sunlight contains much higher levels of ultraviolet radiation than artificial indoor lighting does. Ultraviolet light breaks down proteins within these viruses, rendering them ineffective. Did you get some virus on your hands or clothes? Spending some time in sunlight can reduce their number.
Fighting coronavirus (and any infectious disease) also involves having a strong immune system. Two ways to easily boost your immune system are to get exercise and reduce stress (5). Spending time in nature often achieves both of these goals.
The main thing to avoid during a viral outbreak is crowds of people. And that's usually easily done in large outdoor spaces.
Given all of the above information, it would seem that parks, trails, and other natural open spaces would be the ideal setting for reducing virus numbers both through environmental factors and by boosting the immune system. Visiting these places should be encouraged. Perhaps official statements such as "avoid crowds" would be more accurate and appropriate than the all-to-easy "stay inside".
So let's stay healthy and fight this coronavirus. Go outside and visit a park.