If you’ve ever worked in pharmaceutical manufacturing, biotech labs, cleanrooms, or even food processing facilities, you’ve probably seen pass through boxes tucked between sterile and non-sterile zones, or between a controlled lab space and a dirty utility area. As someone who’s spent the last 12 years as a pass through box supplier—troubleshooting installations, answering midnight calls when a client’s audit is in 48 hours, and helping small startups scale their facilities—one question I get asked more than any other is: “What environmental requirements do we need to hit to make these work?” It’s not just a checkbox list of specs; it’s about keeping your products sterile, your team safe, and your facility compliant. Let’s break this down the way I do with every new client, no fluff, just the real science and practical tips I’ve learned over thousands of projects. Pass Through Box
First, let’s get on the same page about what a pass through box is, because a lot of people new to controlled spaces mix these up with basic transfer hatches. A pass through box is a sealed, double-walled unit with two interlocking doors—you open one to load an item, close it completely, then open the other to retrieve it—designed to prevent cross-contamination between two adjacent areas. That core design is only effective if the environment around it and the way you operate it align with strict environmental parameters. If your surrounding spaces are dirty, or your pass through is installed incorrectly, it’s just a metal box with two holes, and you risk ruining batches of vaccines, cell cultures, or even gourmet food products.
Let’s start with the most foundational environmental requirement: differential air pressure. This is non-negotiable. I’ve seen clients install a pass through between a Grade A cleanroom (the highest sterility level for things like injectable drugs) and a Grade D non-cleanroom on a Wednesday, and by Friday, their Grade A space was failing viable particle counts because the pass through doors weren’t sealed to maintain pressure. The rule here is simple: the zone with the higher sterility (or higher contamination risk? Wait, no—wait, let’s clarify: the zone you need to protect is the one that needs the lower pressure relative to the adjacent dirty zone. Wait, no, let’s get this right. For example, if you have a sterile cleanroom (say, ISO 5, equivalent to old Grade A/B) on one side of the pass through, and a general lab or production area (ISO 8, Grade D) on the other, the cleanroom side of the pass through must be at a positive pressure relative to the adjacent non-sterile space. That means air flows out of the pass through into the non-sterile zone, not the other way around. Wait, no—actually, better to tie it to adjacent zones: the differential pressure across the pass through’s two openings must be in the same direction as the main facility’s pressure cascade. Most controlled facilities have a pressure gradient: cleanest zones (ISO 5) are highest positive pressure, followed by lower clean grades (ISO 7, ISO 8), then utility areas (like waste rooms, garmenting areas) are lowest positive or even negative pressure. The pass through must mirror that: the side connecting to the cleaner, higher-pressure zone is kept at a pressure that maintains the overall cascade, and the side connecting to the dirtier, lower-pressure zone is aligned with that. For example, if your cleanroom is at +10 Pa relative to the adjacent changing room, and the changing room is at +5 Pa relative to the utility hallway, the pass through’s clean side should be at +8 Pa, and the dirty side at +3 Pa, so air never flows backward from the dirty zone into the clean one. I always tell clients to test pressure differential with a magnehelic gauge before even commissioning the pass through. We supply a gauge with every unit we sell, but I recommend calibrating it quarterly, because even a 2 Pa gap can let unfiltered air slip through when a door is opened. And don’t forget: the pass through itself has two internal zones. When you open the first door, the load area inside the pass through is exposed to the first zone, and when you close it, that space should also maintain a small, consistent pressure relative to both sides. If you don’t, you’ll get particle buildup inside the pass through, which then gets blown into the clean zone when you open the next door.
Next, particle count control. This is where things get specific for different industries. For pharma clients working with injectable drugs, the pass through has to be designed to keep particle levels in line with ISO 14644-1 standards. For a pass through between an ISO 5 cleanroom and an ISO 8 area, the internal volume of the pass through can’t have more than 3,520 particles of 0.5µm or larger per cubic meter when in use. How do you get that? Two ways: first, the pass through must have HEPA filtration (for ISO zones up to ISO 5) or ULPA filtration (for ISO 5 and above) installed, so air is constantly circulated and filtered. Second, the surrounding environment can’t have excess particles. If you’re loading a pass through from a dirty staging area with sawdust or lint, those particles will get inside, no matter how good your filtration is. I once worked with a craft brewery client that tried to use a basic pass through between their grain storage area and their sterile filling line. They kept failing particle counts because grain dust was coming through the pass through’s door seals. The fix? We added a pre-filter on the dirty side of the pass through, and installed air curtains above both doors to prevent airborne dust from being drawn in when doors were opened. That small adjustment cut particle counts by 98% within a week. Also, particle counts inside the pass through need to be tested after every load, not just at final commissioning. We recommend a portable particle counter, and training staff to wait 60 seconds after closing a door before opening the next, to let any transient particles settle.
Then there’s temperature and humidity control, which is often overlooked but can wreck products even if sterility is maintained. For biotech labs working with cell cultures or plasmid DNA, the pass through environment has to stay within ±1°C of the required storage temperature. I’ve seen clients transfer a batch of monoclonal antibodies at 18°C in a pass through that was sitting at 24°C because the facility’s HVAC couldn’t reach the small, enclosed space. The antibodies degraded within hours, and that was a $50,000 batch lost. For pharmaceutical products that are sensitive to moisture, humidity control is equally important. For example, powder-based drugs can clump if exposed to high humidity, so the pass through between the dry powder processing zone (30-40% RH) and the cleanroom must be kept at the same humidity level, with pressure differentials that prevent humid air from the adjacent zone slipping in. I always advise clients to integrate the pass through box into their facility’s HVAC system, not treat it as a standalone unit. Even a small pass through (30cm x 30cm x 45cm) can hold 1 cubic meter of air, and if it’s not connected to your main HVAC, it will quickly adapt to the temperature and humidity of whichever zone it’s sitting in. We offer optional integrated HVAC connections for clients building new facilities, and supplementary heating/cooling units for retrofits, which are a lifesaver for older facilities that can’t adjust their main HVAC easily.
Another often-missed environmental requirement is surface cleanliness and compatibility. The pass through’s internal surfaces have to be non-porous, resistant to the cleaning agents you use, and easy to sanitize. But that’s not just about the box itself—it’s about the environment surrounding the pass through’s mounting area. If the pass through is installed between two walls that have porous, paint-based drywall, those surfaces will absorb bacteria and particles, which can transfer to items being passed through. We always recommend that clients line the walls adjacent to the pass through with smooth, non-porous epoxy or stainless steel panels, extending 1 meter on either side of the box, to eliminate that contamination risk. Also, the area around the pass through’s door tracks has to be kept free of debris and spills. If liquid spills on the floor near a door, it can seep under the seal and into the pass through, leading to microbial growth. We recommend installing a small stainless steel sill under each door, sloped away from the pass through, to catch spills and make cleaning easier. For clients working with high-level biohazard materials (like pathogen research labs), the pass through also needs to be compatible with decontamination processes, like vaporized hydrogen peroxide (VHP). The environmental requirement here is that the space around the pass through is vented properly during decontamination, so VHP doesn’t build up to toxic levels, and the pass through’s seals can withstand repeated VHP exposure without degrading.
Wait, let’s talk about microbial contamination specifically, because that’s the big one for clean environments. The pass through’s environment has to support microbial count limits that match the zones it connects to. For ISO 5 cleanrooms, the microbial count inside the pass through should be less than 1 colony-forming unit (CFU) per cubic meter, for ISO 7 it’s less than 10 CFU per cubic meter, and for general lab areas it’s up to 100 CFU per cubic meter. How to maintain that? Regular sanitization of the pass through’s surfaces, but also the environment around it. Staff can’t track in contaminants on their shoes, so the area near the pass through should have proper foot baths or shoe changing stations, depending on the zone classification. I’ve had a client in the medical device industry that was failing microbial counts because staff were walking across a carpeted hallway to reach the pass through, dragging lint and skin cells with them. We installed a stainless steel anti-fatigue mat with a built-in adhesive strip for shoes, which caught 90% of that debris, and their microbial counts hit compliance within two weeks. Also, the pass through’s gaskets and seals are a critical environmental point. They have to be made of a material that doesn’t shed particles or harbor bacteria—EPDM rubber or silicone is standard, because it’s non-porous, easy to sanitize, and resistant to temperature fluctuations. We test every gasket on our units for particle shedding before shipment, because even a small gasket that sheds fibers can ruin a batch of sterile products.
Now, let’s address common mistakes I see, because that’s what actually matters for suppliers like me. A few years back, a mid-sized pharmaceutical company bought three pass through boxes from a discount supplier, and installed them themselves without checking environmental parameters. Within three months, they failed a FDA audit because the pressure differentials between their cleanroom and the pass through were off by 5 Pa, leading to particle ingress. They ended up calling us to come in, test each pass through, adjust the pressure connections, replace the low-quality gaskets, and retrain their staff. The total cost was twice what they would have paid to work with our team from the start. Another common mistake is not accounting for transient loads. When you pass a 10kg autoclaved container through a pass through, it displaces air and particles—you need to make sure the pass through’s environment has enough filtration and air change rate to handle that transient load, so particles don’t get trapped inside. We calculate air change rates for every pass through we sell: for a standard 1 cubic meter pass through, we recommend 20 air changes per hour for ISO 5 connections, which is enough to clear any transient particles within seconds.
At the end of the day, the environmental requirements for a pass through box aren’t arbitrary rules from regulatory bodies—they’re built on decades of experience keeping products safe, teams compliant, and operations running smoothly. As a pass through box supplier, I don’t just sell metal units with doors; I sell solutions that work within your facility’s unique environment. Whether you’re building a new biotech lab, upgrading your pharmaceutical production line, or setting up a food processing facility, getting the environment right around your pass through box is what separates a smooth audit from a costly batch failure, and a safe workspace from a compliance headache.
If you’re planning to install new pass through boxes, or having issues with your current units meeting environmental standards, my team and I are here to help. We work with facilities of all sizes, from small startup labs to large global pharmaceutical manufacturers, to tailor pass through solutions that meet your specific environmental requirements, provide expert installation support, and offer ongoing maintenance to keep your spaces compliant. Reach out to our team to discuss your project, and we’ll help you get the right solution for your needs.
Air Shower References
ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration, International Organization for Standardization.
EU GMP Annex 1, Manufacture of Sterile Medicinal Products, European Medicines Agency.
US FDA Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, U.S. Food and Drug Administration.
Suzhou Jinye Air Purification Steel Structure Co., Ltd.
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