Hacker News Reader

Comment by PaulKeeble | Hacker News Reader

There has been a number of these studies in schools and hospitals showing HEPA filters reduce up to about 2/3rd of infections. We now know many viruses are airborne including Covid and Al Hubbard's studies on how long virus is viable has shown it can be hours and it's helped by high CO2. Air circulation and reducing CO2 makes a big difference as well.

View on HN

Do we have evidence that CO2 is causal or is it still just a correlation? In general, poor ventilation + humans = high CO2, so unless there was a controlled experiment done that's a pretty strong confounding factor.

CO2 in terms of illness is merely a proxy for good air movement and exposure to the outside environment. It has its own impact on mental capacities of course.

> Results showed increasing the CO2 concentration to just 800 ppm, a level identified as well ventilated, resulted in an increase in viral aerostability. After 40 minutes, when compared to clean air, around 10 times as much virus remained infectious when the air has a CO2 concentration similar to that of a crowded room (3,000 ppm).

> Dr Haddrell said: “This relationship sheds important light on why super spreader events may occur under certain conditions. The high pH of exhaled droplets containing the SARS-CoV-2 virus is likely a major driver of the loss of infectiousness. CO2 behaves as an acid when it interacts with droplets. This causes the pH of the droplets to become less alkaline, resulting in the virus within them being inactivated at a slower rate.

https://www.bristol.ac.uk/cabot/news/2024/virus.html

Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk

https://www.nature.com/articles/s41467-024-47777-5

Thanks! This is exactly what I was looking for.

I have never seen a evidence that CO2 is a casual agent of disease (extremely obvious exceptions excluded)

Do you still need HEPA/air purifier if you have decent ERV/HRV?

Maybe.

I have a “decent” ERV. The manufacturer claims that it’s great against air pollution, wildfire smoke, etc, and those claims do not come with numbers. Empirically (as measured by an actual particle counter), it’s crap. It does have real numbers for ventilation rate, and it works great for ventilation. This is a high end ERV, and I see no evidence that any other model is better. And it can’t be: a good, energy efficient filter is large, and all the manufacturers want their devices to be small.

My solution: put a great honking filter in front of it. This increases the lifetime of its crap internal supply filter to effectively infinity (or I could remove it), adds basically no resistance, and reduces supply particulates to effectively zero.

“Great honking” here means a 24"x24"x12" “nominal” HEPA filter with a MERV 8 prefilter. The prefilter costs basically nothing. I expect the HEPA filter to last for years. I would have preferred a not-quite-HEPA 99.5% or 99.9% filter instead, but those are surprisingly hard to buy. Big HEPA filters are easy to buy. Two big MERV 16 filters in series would do the trick, too.

(The filter efficiency math is entirely different for a ventilation system than for a recirculating filter. For recirculation, MERV 13 is fine and MERV 16 is a bit better. For once—and-done ventilation, you want to reduce outdoor crap to your preferred levels in a single pass, and MERV 13 is not even close to good enough in places with pollution or smoke issues.)

Coversely, a poorly filtered fresh air system is fine for reducing air contaminants from indoors but will actively fight against a standalone air purifier if your goal is to reduce outdoor pollutants.

I think using a HEPA air purifier can still be beneficial. ERV and HRV typically have standard filters that may not capture very fine particles, allergens, or airborne pathogens as effectively as HEPA filters do.

Probably depends more on the air changes per hour. You'd have to compare running a filter in your room/house and the air changes it'd go through in an hour compared to your ERV/HRV. My bet is that an ERV is better if they're the same. But, it is much easier to add capacity to individual rooms with a standalone air filter than it is to change your ERV to be higher output.

Reply on news.ycombinator.com