Mold Remediation Blog

How Most Remediators In Central VA, Northern VA & DC Botch Air Pressure

Air Pressure Is One Of The Most Important Aspects Of Remediation.
Unfortunately, It’s One Of The Most Overlooked.


Mold remediation can get pretty complicated. But there is one mistake I see Northern VA, Central VA, and Washington DC remediators make with alarming frequency. And it can have seriously harmful consequences for you and everyone in your home. It all revolves around how mold companies set up the HEPA-filtered Air Filtration Device (AFD) in the containment area. There are two ways to use an AFD… 
  1. As a Negative Air Machine (NAM) to prevent mold spores from escaping the containment during demolition.
  2. As an Air Scrubber to clean the air during the cleaning process.
Unfortunately, I constantly see mold companies using these functions at the wrong times during remediation. And that can be a BIG health hazard for your household. I’ll explain…

When To Use An AFD As A Negative Air Machine (NAM)

Even the best containment barrier won’t stop 100% of mold spores from slipping through. Mold spores can penetrate small gaps in the barrier, when the remediator enters or exits the containment and, most importantly, floats into other areas of your house after walls or ceilings are cut open. 

If the contained area isn’t under negative pressure during remediation, mold spores will stampede through these gaps like it’s Black Friday at Wal-Mart. 

This pressurization is the exact same as isolation chambers in hospitals. Need to isolate someone with tuberculosis or COVID19 so they don’t get the rest of the hospital sick? The hospital puts them in a room that’s under negative pressure.

For proof of how important negative air pressure is, consider this: There are 1,000,000 spores per square INCH of visible mold.1 And the number of fungal fragments can outnumber spores by 320 times.

This is why the ANSI/IICRC S520 Standard For Professional Remediation dictates running the AFD as a NAM during the during the demolition process (a.k.a. tearing out moldy walls and sanding moldy wood). The NAM is placed in a strategic location and we connect a duct to the exhaust of the NAM so the filtered air goes outside (typically through a window). 

Side Note: We could just run the duct outside the containment. The containment will still be under proper pressure and the HEPA filter, inside the NAM, is going to filter out all mold spores and fungal fragments. 

So why don’t we like to? Odors. Even though the air is cleaned, it could smell like mold. See, filters don’t remove odors. And clients don’t like smelling moldy odors. They think there’s something wrong and we’re blowing mold around their house. That’s the only reason we prefer to duct the exhaust outside.

Most importantly, the S520 says the containment should be under -5 Pascals or more of negative air pressure. And the only way to measure the pressure is with a special $1,500 machine called a manometer. 

This is where almost all remediators fall down. Only 1 in 10 remediators in the Northern VA, Central VA, and DC areas put containment under negative pressure. Even rarer (we’ve only heard of 2 other remediators) are companies that have manometers and measure the pressure. At Valor, each lead tech has their own manometer. 

Every time a Valor tech goes to a formal mold class, they’re always the star pupil. Why? Because they bring out their manometer for “show and tell” with the class. 

The instructors are always impressed because we don’t just pay lip service to the S520 standard; we live it every day

My techs don’t realize how good we are until they’re put in a class with their peers from other companies and realize we do things the right way every single time. They always come back from class a little prouder and have a little extra pep in their step knowing they’re better than the rest!

Bottom line… 

When spores go airborne during demolition, the NAM sucks them all up and sends them outside. This negative pressure prevents mold spores from escaping our containment… and floating into other parts of your home. 

Air filtering through the window via negative air pressure.

When To Use An AFD As An Air Scrubber

Only after all visible mold is removed is it safe to switch the AFD to an Air Scrubber. Essentially, this involves removing the duct from the AFD and simply allowing the AFD to run.  When this happens, the AFD cleans (“scrubs”) the air of harmful particles. An Air Scrubber removes at least 99.97% of all particles (mold spores, dust particles, and everything else) greater than 0.3 micrometers from the air that passes through it. To put that in perspective, the smallest mold spore is 1.0 micrometer. If the Air Scrubber is appropriately sized to the room, it will clean the air thoroughly in 2 hours, which is called an Air Change. (Running an Air Scrubber for 48-72 hours like some companies do is pointless—I explain why in this article: Exposed: The Daily-Rental-Fee Ripoff Of “Commercial” Air Scrubbers.)

How Mold Companies Mess Up Air Pressure

I see too many mold companies put homeowners in danger by running the AFD as an Air Scrubber during the entire project… including the demolition phase.  Or even worse: Past clients of one well-known remediation company have told me the company will come in… and open the window during remediation because they want to bring in “fresh” air.  That is possibly the most reckless, ignorant, negligent thing I think a remediator can do. Why? Because there is always a small amount of wind that blows every day. This wind will blow in through the window, put the room under positive pressure… and drive those airborne mold spores into other parts of your home. If the contained area isn’t under negative pressure, nothing is stopping mold spores from escaping the room. To see why that’s unsafe for you and everyone in your home, check out this article for details on the dangers of mold spores. Another issue I see is when an AFD is run as a NAM, it can be done incorrectly or at the wrong time. For example, if there’s a gas appliance like a water heater or furnace, negative air pressure can cause a backdraft of carbon monoxide. Or if the remediator is taking the moldy ceiling out of a room beneath an attic, a NAM would suck who-knows-what out of the attic and into the room they’re cleaning. Either way = not good.

We Do Air Pressure RIGHT

This is why it’s best to choose a mold remediator in Northern VA, Central VA, Southern MD, and Washington DC that understands—and cares about—the science behind air pressure.

We are that mold remediator.

We do everything “by the book.” Our track record is proven. And we take the time to think everything through to create a safe, effective plan of action for removing your mold.

If that sounds like the kind of company you want handling the mold in your home, get in touch today. We would be honored to discuss your project with you.


  1. Cull, Ian; IAQA University, Remediation 103: Engineering Controls – Spores, page 11
  2. Gorny, R.; Reponen, T.; Willeke, K.; Schmechel, D.; Robine, E.; Boissier, M.; Grinshpun, S. Fungal Fragments as Indoor Air Biocontaminants. App. Environ. Microb. 2002, 68, 3522-3531

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