Negative pressure rooms

This has been bothering me for a long time. In the biomedical and nuclear fields (among others), work is often done in negative pressure rooms, to keep the nasties from entering the environment. What I want to know is - how is the air treated? I can understand something like Plutonium machining, where most dust is going to be filterable, but what about Biowarfare? Presumably, even a single virus escaping could be enough to end Civilization as We Know It, so how is the air treated to guarantee that nothing nasty escapes?

The air is HEPA filtered (sometimes multiple times) before exhausting. This will filter out most bacteria, spores, or viruses.

The filters can be treated with UV light or ozone as well for further reduction.

Of course, for really nasty viruses, the best defense is self-contained units that don’t allow the virus to become airborne. The filters are there as a backup.

It’s actually somewhat easier than chemical or radiological decon, since pathogens are quite a bit larger and can be filtered.

First off, I believe you’re comparing apples and oranges here.

With nuclear options the filtering required is fairly simple, and a certain level of leak-through is going to be considered acceptable. The concern is particulate matter getting entrained in the air. The use of things like cyclonic seperators in the outgoing air ducting will reduce entrained particulate contaminants to a minute fraction of what they were to begin with. (Basically a cyclonic seperator makes the gas - air in this case - spin, so that the heavier stuff - dirt, dust, etc all falls off to the duct walls.)

With biologicals, and to a lesser degree chemical, the usual thing I’ve heard is that they add a burn cycle to any air passing out of the hoods, at least, to incinerate anything that might be escaping. But I really can’t say for how they handle the higher level biohazard labs.

That’s also filterable. You need a filter with awfully small pores, but it’s filterable. According to the CDC, exhaust air from a Class III cabinet (used for Biosafety Level 3 or 4 contaminants - level 4 is things like Ebola or Hantavirus) is either filtered through two HEPA filters in series, or first HEPA-filtered, then passed through an incinerator. If I may butcher a perfectly good phrase: no matter how virulent the virus, a furnace is going to seriously cramp its style.

Here’s some more details about how a BSL 4 facility should be set up.

For starters, there are antiviral filters, virus burners (see "Bio Hazard Hood on that page), and so on. I think a good start would be this article about clean rooms.

Also, I have no cites for this, so perhaps others could expand on it or shoot it down as faulty memory on my part, but I seem to recall reading that in most cases, you need a certain number of viruses to infect; less than that and it is highly unlikely that there would be a viable population for a doomsday scenario. Don’t hold me to that, though, I could be wrong!

One redirect question, though - the experience I had with negative pressure rooms was that they accomplished that using massive fans, with huge air flow. My experience with HEPA filters had always been that they always had a relatively low CFM maximum rate of flow, for which they were still effective. Do they make HEPAs that can handle, say 500,000 CFM flow rates? (That’s a back of the envelope guess at how much air was passing through our engineroom back when I was in the Navy nuclear power program.) HEPAs are very effective, but I always thought they were of limited utility for larger air flow situations.

So a 10’ diameter fan would use 25 of these, handling a nominal flowrate of 37,500 cfm. How many fans would it take to produce 500,000 cfm?

It may well be that I over estimated the volume of the enginerooms (for one thing I ignored the volume of any equipment in my back of the envelope calculations), but we had fan rooms with an aggregate volume (and often very little free space) almost as large as the enginerooms themselves. IIRC there were three fans, per engineroom, but I can’t swear to that. It’s been 12 years, and the fans were never something that interested me. I just remember that the airflow was set for the enginerooms to (nominally) completely replace the air volume in the engineroom three times a minute.

Oh, a clarification: I don’t believe our engineroom exhaust ventilation was HEPA filtered. the reason for the negative pressure was to keep contamination, in the event of an accident, away from the rest of the ship.

I appreciate all of the responses.

I used to work in the Semiconductior industry, so I’m familiar with HEPA filtration. I wasn’t aware of the incineration chamber (although I had always thought something like that would be necessary). I wonder if these rooms (and the air handlers) are checked for leaks after they’re built (and how would that work - particle counters on the exhaust?)

For less critical filter systems (say, removing chemical pollutants), the usual way to check for leaks is to check the pressure drop at various points along the air path. If the pressure changes, then something isn’t working like it used to.

I’m not sure about checking for virus-sized cracks, though.

This may be a stupid question, but why would you have such a high airflow rate through a room with potentially really bad stuff in the air? It’s not like the Plutonium or Ebola needs a fresh air supply, is it?

My own experience with negative pressure rooms was that they arranged that pressure differential precisely by taking advantage of that huge airflow rate. It may be possible to arrange it through some other method, but I can’t help thinking that the simplest way to arrange it is through the kind of ventilation I’d experienced.