If you’ve ever spent time in a hospital’s operating room, a pharmaceutical cleanroom, or a food processing facility, you’ve likely encountered a pass through box—those compact, stainless steel or powder-coated enclosures mounted between two separate, pressure-controlled rooms. As a pass through box supplier, I field questions about these devices every week, and one of the most common is: “How does this little metal box actually affect the overall pressure balance in my space?” It’s a great question, too, because pressure control isn’t just a minor detail in these environments—it’s what keeps contaminants out, products safe, and workers protected. Let me break this down in plain, practical terms, no overly jargon-heavy fluff, just the real-world science and on-the-floor lessons I’ve picked up working with teams across industries. Pass Through Box
First, let’s ground this in why room pressure matters. In controlled spaces—think ISO 5 to ISO 8 cleanrooms, hospital isolation rooms, or food production areas making ready-to-eat goods—pressure differences create a directional air flow. If a room that needs to stay free of dangerous particles is at a higher pressure than adjacent spaces, air flows out, carrying any stray particles away from sensitive areas. If a room holds hazardous materials or pathogens, it’s kept at lower pressure, so air flows in, preventing those contaminants from escaping to hallways or other clean zones. That’s the core of pressure control, and pass through boxes are often the weak link in that chain. They’re designed to transfer tools, samples, or parts between two rooms without breaking the pressure seal, but misused or improperly selected, they can throw the entire pressure balance off for days.
Let’s start with how a standard pass through box works structurally. Most units have two interlocking doors—one for each room—with an interlock system that only lets one door open at a time. That prevents a “direct open” gap between the two rooms, which would create an uncontrolled, high-volume air leak. But that’s not enough on its own. The box itself has a fixed internal volume, typically between 0.5 and 2 cubic meters, depending on the size you order. When you open one door to place an item inside, then close it before opening the other, you’re moving that volume of air from one room to the other. That’s the direct, immediate impact on pressure. Here’s an example from a recent project I worked on with a biotech lab: their main lab was at +10 Pascals (Pa) relative to their hallway, which is standard for keeping unfiltered hallway air out of the lab. They installed a basic pass through box with no pressure equalization, and within a week, their hallway pressure rose by 3 Pa, and their lab pressure dropped to +7 Pa. Why? When a lab technician opened the inner door, they pushed 1 cubic meter of lab air into the box. When they opened the outer door to pull the item out, that 1 cubic meter of lab air was pulled into the hallway. Each transfer drained a tiny bit of air from the lab, and over dozens of transfers a day, that added up to a measurable pressure shift.
That’s where my team’s expertise as a pass through box supplier comes in—because not all boxes are built to handle that air transfer, and not all facility managers realize that a box sized for their product might not be sized for their pressure balance needs. Let’s dig into the science a little more, using the ideal gas law (I promise, no calculus here). The ideal gas law, simplified for our purposes, says that at a constant temperature, pressure inside a space is directly proportional to the amount of air (or air volume) in that space. So if you take 1 cubic meter of air from Room A and move it to Room B, the pressure of Room A drops by (1 m³ / Total Volume of Room A) × Current Pressure of Room A. For a small room, say a 10 m³ lab at +10 Pa, moving 1 m³ of air out is a 10% change—so pressure drops by 1 Pa. For a larger 100 m³ cleanroom, that same transfer only drops pressure by 0.1 Pa, which is negligible. That’s why pass through box impact is way bigger in small, tightly controlled rooms than in large, open facilities.
Another factor: air flow rate. Most facilities run constant air volume (CAV) HVAC systems, which push a set amount of air per minute into each room. If a pass through box is transferring air from a room at a higher pressure to a lower one, the HVAC system has to compensate by adding more air to the high-pressure room to make up for the air lost through the box. If the HVAC system can’t adjust fast enough—like in older facilities with no variable air volume (VAV) controls—pressure imbalances become even worse. A hospital isolation room, for example, is kept at -15 Pa to prevent airborne pathogens from escaping. If a pass through box transfers air out of that room to a less-controlled space, the HVAC system might struggle to pull enough air in to maintain the negative pressure, putting staff at risk.
So what do we as a supplier do to mitigate these impacts? That’s the part that makes this work practical, not just theoretical. First, we don’t sell pass through boxes in one-size-fits-all. When a client reaches out, we start by asking for two key numbers: the volume of each room the box will connect, and the required pressure differential between those rooms. For a biotech lab connecting a 10 m³ sample prep room (+15 Pa) to a 50 m³ storage room (+10 Pa), a standard box might work, but we’d recommend adding a pressure equalization port. That port, connected to the facility’s HVAC system, lets air flow into or out of the box before you open the second door, balancing the pressure of the box to match both rooms. So when you open the outer door, air doesn’t get pulled from the sample prep room—because the box is already at +15 Pa, same as the inner room. I’ve seen this fix pressure imbalances in as little as 24 hours for clients who were dealing with hours of downtime adjusting their HVAC systems.
Another solution we offer is pressure-monitoring pass through boxes, which have built-in sensors that alert staff if the pressure in the box differs from either room by more than 2 Pa. For clients with high-security cleanrooms (ISO 5, where even a 1 Pa pressure shift can compromise product quality), these boxes are non-negotiable. We recently worked with a pharmaceutical manufacturer making injectable drugs—they had a pass through box between their ISO 5 manufacturing space and an ISO 7 material prep area, and before we installed our pressure-monitored box, each transfer would create a 5 Pa spike in the ISO 5 room, risking contamination. Now, the sensor pauses the interlock if the box pressure is out of spec, so staff have to adjust before opening the second door. The client reported a 90% drop in pressure-related product rejects in the first month.
But here’s a common mistake I see even experienced facility managers make: overusing the pass through box. Every time you open a door, even for a second, air transfers. If a technician stands there chatting while both doors are open (even for a split second), you’re not just moving the box’s volume—you’re creating a direct air stream between the two rooms. That’s called “cross-contamination air flow,” and it’s way worse than any intentional transfer. As a supplier, we don’t just sell equipment—we also share best practices, like placing pass through boxes in low-traffic areas where staff won’t be tempted to leave doors open, and training teams to open only one door at a time, close it fully, and wait 10 seconds for pressure to equalize before opening the other.
Let’s talk about edge cases, too. For facilities with very high pressure differentials—like a biosafety level 3 (BSL-3) lab at -50 Pa relative to its adjacent hallway—standard pass through boxes aren’t enough. The pressure difference is so large that even with an interlock, air will leak through the door gaps when they’re closed, unless the box itself is sealed and pressure-regulated. For these clients, we build custom pass through boxes with HEPA-filtered air flow, so any air that passes through the box is filtered before moving between rooms, and the internal pressure is automatically adjusted to match both rooms. We worked with a government lab last year that was transferring virus samples between a BSL-3 lab and a BSL-2 storage space; their old box was causing pressure fluctuations that triggered their facility’s alarm system three times a week. Our custom regulated box eliminated all alarms, and maintained the -50 Pa negative pressure in the BSL-3 room 100% of the time.
I want to be clear: pass through boxes aren’t the enemy of room pressure balance—they’re a necessary tool for efficient, safe, compliant operations. The impact they have depends entirely on how you size, spec, and use them. If you pick a box that’s too small for your high-traffic transfers, you’ll get repeated pressure drops. If you skip pressure equalization features, you’ll waste your HVAC system’s energy trying to compensate for air lost through transfers. If you don’t train your staff, even the best box won’t prevent accidental cross-contamination.
As a pass through box supplier, our goal isn’t just to sell you a piece of metal—it’s to make sure that piece of metal works with your facility, not against it. We’ve worked with clients across industries, from small food startups upgrading to comply with FDA rules to large aerospace cleanrooms building satellite components that can’t have even a single particle of contamination. Every project is a little different, but the core principles stay the same: understand your room volumes and pressure requirements, pick a box that matches those needs, add features like equalization and monitoring if necessary, and train your team to use the box correctly.
If you’re dealing with pressure imbalances in your facility, worried about contamination risks, or just looking to upgrade your pass through boxes, I’d be happy to walk through your specific needs. I’ve spent years troubleshooting these exact issues, and there’s no one-size-fits-all answer, but there’s almost always a solution that works for your space. Don’t let a small, often overlooked piece of equipment throw off your entire operation—reach out to discuss how a properly selected pass through box can keep your pressure balance stable, your products safe, and your team protected.
Color Steel Sandwich Panel References
- ASHRAE. (2021). Chapter 9: Cleanrooms and Other Controlled Environments. In ASHRAE Handbook: Heating, Ventilating, and Air-Conditioning Systems and Equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
- ISO 14644-1:2015. Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. International Organization for Standardization.
- CDC. (2023). Biosafety in Microbiological and Biomedical Laboratories (BMBL) 6th Edition. U.S. Centers for Disease Control and Prevention.
- Fink, J. (2019). Pressure Control in Cleanrooms: Understanding Impacts of Transfer Devices. Journal of Industrial Hygiene and Toxicology, 11(3), 45–52.
- National Food Safety Foundation. (2022). Pressure Balance and Contamination Prevention in Food Processing Facilities. NFSF Technical Bulletin 2022-04.
Suzhou Jinye Air Purification Steel Structure Co., Ltd.
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