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We are sure by now you have heard that the nasty virus COVID-19 mostly spreads through air. If this is news to you or you want to dig deeper to understand how exactly do we know that to be true, we can highly recommend “FAQs on Protecting Yourself from COVID-19 Aerosol Transmission”. This is an excellent document – work done by Prof. Jose-Luis Jimenez, Prof. Shelly Miller and many other (in total 10) world renowned scientists in related fields. This is a summary of the evidence that anyone can understand and also backed up by of citations for all their claims. If you are more of a listener, then we would recommend that you check out our webinar episode “The Science of COVID-19 Aerosol Transmission – an Interview with Dr. Jose-Luis Jimenez”, where we go into depth about these questions.

Of course, the key takeaway from this is that fresh air is our only weapon against airborne COVID-19. That is why everyone lately has been obsessed with finding ways to provide sufficient air exchange in-doors. And we also have played our part in it. To be honest some of it can be attributed to luck – we had the right device at the right time – the wireless battery powered air quality meter Aranet4. However, because SAF Tehnikas’ R&D team relentlessly imposes high standards not only in technical equipment quality, but also for its ease of use and visual aesthetic, Aranet4 quickly became the favorite device to many of the world’s top aerosol scientists working in the field. This helped us tremendously to get serious recognition and take part in many interesting indoor air quality control projects.    

Initially we focused on the most challenging places for optimal air exchange – schools, kindergartens, day-care centers and universities. Any educational facility is the worst-case scenario, because you have a lot of people in a single room, many of them speaking, producing potentially infectious aerosols. In many cases these facilities cannot afford a sufficient mechanical air ventilation system, so their best option is to rely on devices like Aranet4 to tell them when to open the windows.    

There are of course many other public places that face similar challenges – museums, libraries, cafes, bars, restaurants, supermarkets the list goes on and on.    

But after working with all these buildings we realized that we may have overlooked public transport and the air quality issues faced there. There are of course studies done pre-pandemic of CO2 levels inside busses and other public transport, and the results differ location by location.    

We wanted to look at our own public transport system here in Riga, Latvia, where Aranet is manufactured, and our head office is located. How safe are the trams, busses, trolleybuses and trains, that many of our fellow citizens must use daily, and what can we do to make them safer?    

Now we must emphasize that this is by no means a thorough scientific study. Our goal here is simply to assess the situation and perhaps inspire public transport companies elsewhere in the world, to take this seriously, and consider easy but helpful actions that can be taken for passenger safety, like installing CO2 monitors.    

On March 24, 2021 we put out a tweet for our local community, inviting anyone who wants to become a citizen scientist and help us measure CO2 concentration in different public transports, during different times of day. In exchange each of them received a beautiful Aranet4 device to do the measurements with which they got to keep afterwards. Not a bad deal for a couple of bus rides.  




So, we had 8 volunteers who spent a total of 30 hours and 2 minutes collecting 1802 measurements – one measurement each minute. This was done in 4 types of public transport:

  • 195 measurements in trams
  • 1093 measurements in buses
  • 248 measurements in trolleybuses
  • 266 measurements in trains

  At times, the volunteers noted how many people are currently inside the transport.    

In all the cases an unsurprising trend was observed – the more people inside, the higher the CO2 concentration. We found that the safest public transport from airborne disease transmission point of view are the city trams. In the graph below we can see the CO2 reading dependent on the number of people present in the tram:  




As you can see, only a single measurement was noted above 1000ppm, which most sources cite as the danger level for airborne COVID-19 transmission level risk. On average every new passenger adds an extra 7ppm. Again – this is a heavy approximation of an otherwise complex and dynamic system, but it illustrates the tendencies well enough. The explanation for this trend can be speculated as “Rigas Satiksme” operates 2 kinds of trams – the new ones have large volumes, without separation between sections. And the older ones, well… Let us just say that there are plenty of openings where fresh air can get in – intentional and not.    

So, if you have made it this far you are probably wondering which is the worst public transport to be in a pandemic. And that really depends on how you define “worst”.    

Busses got the record of the highest values measured – several above 2000ppm, however some of that can be attributed to pure statistics – there simply were more measurements in buses. But it is worth looking at the graph just as a comparison to the tram situation:    



Busses are typically much smaller and much more isolated from the outside air than trams. On average each new person that enters the bus increases the CO2 level by 30ppm. Trains and trolleybuses fall in the middle of both if we look from the same point of view.    



However single point measurements in these transports do not tell the full story. The contamination of the air is only one variable in this risk assessment formula. Time spent in this contaminated area is a factor of a far greater importance. You could quickly walk through a contaminated room, and your risk would be far less than if you spent minutes or even hours there. So, this is what we did next, we took all the 1802 measurements done in the public transport, each as a one-minute sample, and looked at what fraction of total time in a given transport is spent in an environment with the CO2 concentration higher than a given value. Most sources (REHVA, etc.) recommend keeping CO2 concentration values below 800ppm to consider indoor spaces safe from airborne COVID-19 transmission, and anything above 1000ppm is already critical and dangerous for your health. The below figure illustrates this analysis for the 4 types of transport we investigated:    



From this perspective it seems that the trolleybus has the highest risk of airborne COVID-19 transmission. More than half of the time in both the buses and the trolleybuses people spent in >800ppm concentrations, and almost half of the time was spent in >1000ppm in trolleybuses. We could extend the levels and see that quite a significant portion of time is spent in concentration levels >1400ppm. This is the level at which besides the contagion risk, our cognitive abilities decrease by half, and this in turn increases the chances of road accidents. But that is for another story.    

And again, please take everything you read here with a grain of salt. This is not a proper scientific study, nor do we want to pretend that it is. The goal of this article is not to prove nor disprove that public transport in Riga is pandemic safe.    

But we do hope to inspire both our local public transport providers and others all around the world to investigate ways of how to improve the safety of their staff and passengers. But you can only improve what you can measure. And if you are a concerned citizen, get yourself a CO2 monitor, take it with you wherever you go, and understand which places you visit during the day are safe, and which are not.     The easiest, most hassle free, best price/performance product for this purpose is Aranet4. You get a portable wireless hand sized device, which fits neatly in your pocket. See the current CO2, temperature and relative humidity values directly on a sleek e-ink display or connect it to your phone via a Bluetooth app to see historical readings. Cast it to a screen in your restaurant, bus, university or any other public space to let your visitors know, that your facilities are safe.    

And don’t just take our word for it – Aranet4 has a great track record being endorsed by world leading scientific experts and we’ve been featured in the news:

On our website you will find our wall of fame, with highly respectable organizations who have chosen Aranet4 to ensure the safety of their air quality, like the ones below:    




So why not give Aranet4 a go yourself? Order it now either directly from our web shop or on Amazon. Or if you want to become our partner to distribute Aranet4 in your local market, contact us directly at    

Take care for the air that you breathe! Choose Aranet4.

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