Comment by cfcf14 | Hacker News Reader

Comment by cfcf14 | Hacker News Reader

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I hadn't considered how bad public transport might be in terms of ventilation, but it makes sense. I wonder how bad the London tube might be on a busy day?

It's unfortunate that research on the impact of Co2 concentration on cognitive function and general wellness is so inconclusive (ie: effect size of high concentration on most tasks seems small), when subjectively it feels really high.

Some years ago I did some datascience work with a housing association wherein they installed high-frequency sensors in various rooms of housing units across the country (with the explicit consent of the inhabitants, data anonymized) with the intent on understanding how building properties and use correlated with the sensor data. In general there was (and still is) a lack of understanding how to model the relationship between things like building material, climate, occupancy, tenant behaviour and negative outcomes like mold growth, fuel poverty/health outcomes, and so on. I mean - I'm not saying there's no understanding, but it's definitely a field where more work is needed.

Anyways, one of the things I found most shocking is that in many of these flats, Co2 concentrations would get mind-bogglingly high in bedrooms during the months where tenants would likely have the windows closed (this is something you can infer by temperature gradients and heating usage). By mid-way through the night, some individuals were in rooms with 4k+ co2 ppm. A common side effect of breathing air with such a high concentration is waking up feeling groggy, disoriented, or with a headache. But what are these people supposed to do - many of these flats were low-income housing, so they're badly insulated to begin with, and they probably can't afford the heating bills of keeping windows open for ventilation.

Sorry for the long story - but the point is that there are many negative health externalities associated with poor ventilation in buildings and interior spaces, and at least we're starting to get attention on the problem now with covid. Hopefully this leads to improvements across the board.

London’s tube is particularly bad because of its history running steam trains, of which the residue is still present on walls and ceilings. The Tube is also poorly ventilated due to its age (when this was less considered) and the fact that vent shafts open into places that also have bad air quality.

NYC performs relatively better, it sits close to the surface with frequent vents. There have been simulated bioterror attacks that show the threat in the subway would dissipate quickly due to the ventilation.

The article mentions a bus. I would expect the bus to ventilate more poorly; trains have many doors per car and they simultaneously open for the entire stop in the station, but a double decker only has two doors, the back open to let people off and closed shortly after that’s done, and the top deck has no doors at all.

I'm not sure there's a significant difference between depths of most of the London and New York subway systems.

Most of the London Underground is cut-and-cover tunnels sitting just below ground level (2-6m). The Thames Tunnel is 24m below ground level. The deepest station is Bull & Bush, Hampstead, at 57m (180ft) underground:

https://londonist.com/2015/10/how-deep-does-london-go

https://londonist.com/london/transport/how-deep-is-your-comm...

The NYC subway also uses extensive cut-and-cover routes, though deeper bores exist. The deepest station, at 191st St. (LRI) is 180 ft. (54m) below ground. About the same as the London Underground.

https://untappedcities.com/2013/06/26/deepest-highest-subway...

https://www.nycsubway.org/wiki/Chapter_14:_Engineering_Featu...

There's also difference in ventilation design. A lot of the tube relies on the piston effect for ventilation.

NYC's subways are underneath major roads and nearly always features extensive sidewalk venting for the entire length, as depicted in the iconic photo of Marilyn Monroe. https://img.atlasobscura.com/EEahxLmWgG-xahr-cXyJ1_TFUKch_pn...

Where "piston effect" is the passage of trains through the tunnels forcing out air (ahead) and drawing it in (behind), correct?

AFAIU that's a major component of any subway's ventillation.

Yes.

The difference is that London relies solely on the piston effect. NYC has the subway vent grates on nearly every block where a subway runs. Modern systems generally also use HVAC.

I am extremely skeptical of your claims re: London.

Steam hasn't operated on the Underground in 115 years, and only ever on a small minority of lines.

I doubt it's because of steam train residue, but when I was on the Underground it turned all the mucus in my nose black for a day every time I rode it. From searching around, this seems to be a common occurrence for visitors. Air quality there is severely compromised. There's also the health and safety problem of its steadily rising temperatures, about which nothing appears to be being done or even considered.

They are trying to install cooling at several stations where they deem it possible.

The problem is that, generally speaking, Central London lacks spots where you could just punch a ventilation shaft downwards, and the stations are too tight to accommodate modern HVAC otherwise.

On the public transit point: fun fact, BART (a large SF/Bay Area transit system) trains replace all of the air in each train car every 70 seconds, which is a much higher rate of airflow than most other public spaces. Since they also use MERV 14 filters, there may be a disconnect between CO2 readings on BART (or other transit systems with aggressive filtration) and the true risk of catching a respiratory virus.

Does the AV help?

(Assuming you mean AC) - HVAC systems are generally designed to keep air flowing through the building optimally, which may include control systems for attenuating fans, exterior building air intake, and whatnot as a function of sensor measurements. I'm not sure how often CO2 is taken into account during building design or operation to be honest, aside from rough targets on designed occupancy rates and volumetric estimates of airflow.

In the UK, most buildings are naturally ventilated. There's no mechanical ventilation moving air around automatically, you gotta open your window if you want airflow. In the winter this generally puts people between a rock and a hard place: either you are slightly cold and sitting in a stuffy roof, or you open a window and are REALLY cold and also go broke from the energy bills :P

What's AV?

I assume it's meant to be AC

Just in case, to answer the question: The vast majority of private, let alone public housing in the UK have no AC.

that does not answer the question

To make explicit: There is no AC therefore the AC doesn't help.

I considered the original question to mean: "Does / would the use of AC help?"

to which tangential, interesting information was provided but left the original question unanswered.

Are we to assume literally nobody has an AC nor is it possible for AC to even exist in those situations? Because if it helps, then maybe something productive can come out of this conversation where we can aim to improve some of these living conditions through use of AC's - which would kill 2 birds with one stone.

It seemed like a genuine question that should be actually answered. I want to assume "yes" they would help, but I'd be guessing.

AC alone wouldn’t make any difference whatsoever. You need a modern HVAC system that exchanges air with the outside with something like an ERV or HRV. And even before doing that you have to make sure the building is sealed and insulated properly so the air is going where you need it to go. It’s actually a fairly complicated problem that starts with the design, building materials, and construction processes of the building itself.

To be precise, there is AC in almost every home in the UK - approximately two-hundread-and-forty volts AC!

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