Abstract:
An experiment is designed to assess the relative affect of temperature on airborne disease transmission in humans.
Background:
Respiratory viruses are known for their high degree of seasonal variation, which is observed globally but a detailed explanation has so far been lacking. Let’s look at some data, here’s USA seasonal influenza numbers;
For the year 2008-09, influenza peaks in February. Influenza B (in green) peaking slightly later.
The question is whether these different infectivity levels correspond to different transmission rates caused by differing humidity, temperature and light-exposures inherent to the different seasons. Or whether different patterns of human behaviours are responsible. These include hot weather causing improved ventilation by people opening, more time spent outdoors, higher vitamin D levels etc. It’s possible both are important.
The goal is to investigate the affect of air temperature on viral aerosol transmission while keeping the temperature and humidity fixed for the existing and new host. In this way the significance of the external environment can be assessed directly.
The hypothesis is the transmission immune system is weakest in cold, dark and wet conditions. Presumed, if dehydrating mucus locks in pathogens and prevents infection, this process will be highly influenced by humidity, temperature and light exposure. It may be we naturally dislike these external environmental conditions because this is when we are most vulnerable to airborne respiratory pathogens…
The Experiment
On the left is a human infected with a respiratory disease (eg influenza A, SARS-CoV-2, coronavirus OC43) which is our transmission source. They are instructed to talk so as to produce virion-containing aerosols. Air is drawn into a mixing chamber, mixed to ensure an equal concentration of infectious aerosols, then channelled off through three identical tubes. The tubes are horizontal to ensure equal settling of aerosols, and the 3 hamster areas need to be as identical as possible, with hamsters arranged to give as close to an equal exposure to aerosols as possible.
What are the hamster areas?
The hamster areas allow the infectivity of the airborne particles to be measured directly. Air enters the hamster area and is split equally among a statistically significant number of isolated hamsters (say, 50). At the end of the experiment, and after a suitable time delay, the proportion of infected hamsters are counted.
Airflow throughout the experiment needs to be as laminar as possible, if flow becomes turbulent after the air is split then aerosols will travel different distances.
Because of the unknown impact of light and the need to repeat the experiment in different laboratories it’s recommended to either carry out the experiment darkness, or to accurate record the light levels across the apparatus.
The Experiment
Tube 1 is the control. There are two identical air heater/cooler grids, that heat or cool passing air as required, and are inactive in this part of the experiment. This shows the base level of infection at short range and aerosol concentration, a fixed distance from the source.
Tube 2 contains an additional length of 1m tube. Cooling/warming grids are present but inactive.
Tube 3 contains an additional length of thermally insulated 1m tube, with air cooled for 1m, then rewarmed to room temperature. Rewarming is required to avoid the lower temperature modifying hamster behaviour.
Experiment temperature, humidity, and time length is recorded.
The goal is to measure the proportion of infected hamsters in each hamster area.
Interpreting results:
No significant infections in any hamster areas. Modify experiment to improve transmission;
- Shorten tubes between hamster areas and infection source.
- Prolong length of time of experiment.
- Increase fan speed to increase air speed.
Almost all hamsters infected in all hamster areas. Modify experiment to reduce transmission;
- Shorten length of time of experiment.
- Reduce fan speed to reduce air speed.
Approximately equal number of infections in each hamster area. Neither distance nor air temperature significantly effects transmissibility.
Proportion infected hamsters in A > proportion of infected hamsters in B ~= proportion of infected hamsters in C. Temperature makes small to no difference to transmission, but distance/time from source does.
Proportion infected hamsters in A ~= proportion of infected hamsters in C >> proportion of infected hamsters in B. Cool air makes a significant increase in transmissibility, and social policy can be modified to reduce infection, eg increasing mask wearing, social distancing in outdoor cold environments.
Limitations of the Experiment
The results would need to be repeated several times to draw firm conclusions. We are assuming that the hamster/animal model of resisting infection by clearing infected mucus before it can contact vulnerable airway epithelial cells is the same for hamsters as it is for humans.
Future Research
Repeating the experiment with different temperatures would allow us to build a new understanding of transmissibility as a function of temperature, with far reaching implications for social policy.