Reply To: Question about Airborne
Hi Jaap and Ilse,
As you can imagine, there are many factors that influence this and it is highly complex topic. There is still a lot we don’t know about this.
You have mentioned a few factors already, so I will focus on those.
Weather: More respiratory viral infections occur during the winter months, compared to the summer. There are several factors which are at play here. Cold air has an effect on the lining of our respiratory tract and our immune system, which appears to make us more susceptible to infection. There is also less UV radiation, which damages the viral particles. In general, viral particles are more stable (less likely to be damaged) at low temperatures, just like your food stays fresher when you keep it in the fridge. Air humidity also plays a role. A very low humidity, respiratory droplets dry out very quickly. This results in smaller droplets with low effective salt concentration, that can remain airborne for longer. Very high humidity (such as seen in tropical climates) can also help a virus survive, because the respiratory droplets remain intact and conditions such as temperature and salt concentrations remain very similar to the human body. At intermediate humidity, viruses are at a disadvantage, because the water evaporates out of the droplets they sit in, leaving salt behind, which can damage the virus. In general, respiratory viruses have it better in cold climates, but as we have seen during the pandemic, they can also spread very effectively in tropical climates, such as Brazil at the moment. Lastly, as you are probably aware of yourself, human behavior changes with the weather, and this can also have a massive impact on viral transmission.
Distance is a much easier one: the greater the distance between persons, the lower the risk of transmission.
Obstacles: Similar to distance, obstacles in the form of facemasks can make it harder for a virus to spread.
Indoors/Outdoors: This is also influences by the weather of course! Apart from this, there are several factors to consider. In general, there is more space outdoors, so social distancing is easier. Air circulation is also better, which means that infectious droplets that become airborne are diluted more quickly. This has lead many people to believe that, as long as you are outside, you don’t have to mind the rules that much. This is not true however, because airflow (i.e. wind) can be very unpredictable. What if you are standing downwind from someone who is infected? Under the right circumstances, viruses can still spread very efficiently outdoors. The same principle applies indoors. Some people have suggested that we should focus on having better ventilation indoors. However, if an open window or a ventilator is blowing air from one side of the room to the other and there is an infected person sitting close to that door or ventilator, that can increase the risk of transmission. The safest way to ventilate a room is by blowing clean air straight down from the ceiling. This way, the infectious droplets are blown downwards, where they are less likely to reach somebody’s mouth or nose. This is how hospital rooms and laboratories are ventilated. Things like air conditioning that many buildings in warmer climates have, also change temperature and air humidity, which can increase the risk of indoor transmission there.