As a consultant in the compounding pharmacy industry I get a lot of calls and a fair amount of interactions, which often are discussing or demystifying things that people have heard. In todays age of social media, discussion forums, conferences, and a relentless circle of media in our lives, it is no wonder the “whisper game” is alive and well with USP chapter 800. Below is a discussion and dissection of two different myths that came across that I wanted to discuss.
Some months ago I was speaking about for a state pharmacy association about USP 800 facility and engineering controls as it relates to USP chapter <800> compliance and one of the questions, or statements rather was something like this, “My consultant told me that I could a buy hood with redundant HEPA filters and I don’t have to externally vent the room.” I told them to refer to Table 2 under section 5.3.1 and they would see that the nonsterile compounding room still must be externally vented. That same person stopped me at lunch and asked me the exact same question because they decided to text their consultant about my response. Again, the consultant confirmed the recirculating redundant HEPA hood was somehow magically creating negative pressure in the room.
Let me drop some sciences on you if you believe that other consultant. When a “hood” recirculates back into a room, the hood is considered “room neutral” and does not affect the overall pressurization of the room. Let me explain this one step further… the containment primary engineering control (C-PEC), also known as a powder hood, or the balance enclosure, or the class one BSC, or the containment ventilated enclosure (CVE), or whatever you want to call the “hood’ pulls air from the room through the face opening in front of the hood. That volume of air entering the face opening is equal to the volume of the air being pushed out of the top exhaust port. It is a one to one equation, which is always equal, which creates a neutral pressure relationship to the room.
Let me back up the science with math. If the face opening of the 4’ wide powder hood is 48” and the height of the face opening (where you stick your hands inside to compound) is 10” tall, then we multiply 48 x 10 = 480 square/inches of face opening. We take 480 sq/in and divide by 144 (the number of square inches in a square foot = 12” x 12”) and we get 3.33 square/feet of face opening. The certifier then measures the face velocity at 80 feet per minute (fpm) with his handy dandy anemometer and writes in on a certification report. If you then multiply 3.33 x 80, that equals 266 cfm. The “CFM” is the amount of air being pulled into the face opening of the powder hood, which I guarantee is the same amount of CFM being exhausted out of the top, which is recirculated back into the room. That is a one to one relationship and does NOT somehow magically create negative pressure in the room.
Now let us take these negative pressure myths one step further. I received a call just the other day and the pharmacy owner said, “I called our state board of pharmacy” and they told me I only had to connect the hood, but not the room. This statement may or may not be true so let us prove it with math yet again. The “Table-2” I referenced earlier from USP <800> states that it is preferred if the C-PEC is externally vented. I will mention my preference for single HEPA hoods connected externally versus redundant HEPA hoods recirculating later down this post.
Here is some math to consider. You have a small community pharmacy doing 10 to 20 compounded hazardous scripts per day, so you opt to allocate a small 10’ wide by 10’ deep room. Without giving you the entire room calculation formula (see my white paper titled “Facility and Engineering Controls Using USP <800> Guidelines”), trust me that the minimum nonsterile air change per hour requirement is 12, which equals around 200 CFM (remember “CFM” is airflow) for the entire C-SEC. In this scenario if you connect the C-PEC in the previous example then you are exhausting 266 CFM. 266 CFM is greater than the required 200 CFM, which equals around 18 air changes per hour (ACPH), so your C-PEC is exhausting all the required air. Now there is no need for a low wall exhaust in the C-SEC because the “hood” is doing all the work. Problem solved.
What if your pharmacy is doing more hazardous drug (HD) compounded scripts, say 50 a day, so now I need at least two technicians and two hoods. At this size you also have multiple pieces of equipment like mills, electric mortar and pestles (EMP), maybe a couple of hot plates, larger sink and dishwasher, computers, etc. and my room is 15’ wide X 25’ deep. The minimum number of air changes for a nonsterile compounding room is still 12 ACPH no matter what size your pharmacy is. At this size floor plan, you need a minimum of 675 CFM to equal 12 ACPH. Two hoods connected at 266 CFM each is only a little over 500 CFM of exhaust and you need probably 800 CFM to create negative pressure. Negative pressure is created by exhausting more air out of the room than what is supplied at the ceiling diffusers. It is just that simple of a concept. Now you need at least one low wall exhaust to make up the difference between 500 CFM and 800 CFM. So is this two-hood scenario both the C-PECs and the C-SEC are externally vented.
I’m sorry it took an extra two paragraphs to explain that the state board of pharmacy cannot simply make an assertion that “only the hoods need to be connected” when they do not know the totality of the pharmacies room size, business model, and number of hoods to be accounted for. My fear moving forward with an actual implementation date is that state boards of pharmacy are going to be completely unequipped to understand simple engineering concepts that are at the core of USP 800. I do not use the word “simple” concepts to be dismissive to board inspectors, so maybe that came out wrong. But I have to say there are going to be pharmacies that implement the standards wrong, which will also incur board inspectors to interpret the standards wrong. Maybe its not so “simple” after all! Touché’.
Externally Vented C-PECs
Many moons ago I used to design and sell “engineered containment” which is just a fancy name for “hoods.” These simple negative pressure boxes are fickle animals, which means if a room door opens, or someone walks by, or the HEPA filter loads, or simply weird stuff happens then the C-PEC tends to fluctuate. Even though the blower is a constant volume it does not mean the C-PEC is always consistent. Here is the point, if the C-PEC is “externally vented” to an exhaust blower pulling the air out of the building, then the room’s negative pressure is prone to fluctuation as witnessed on the room’s pressure gauge, especially if the pressure gauge is digital and giving real time measurements.
My fifty-cents worth of advice here is, if your state board of pharmacy will accept the Table-2 carve out known as “redundant HEPA” hood recirculated back into the hazardous drug room, then buy those and let them recirculate. Yes those hoods are more expensive but it is better to balance a containment secondary engineering control (C-SEC), which is the compounding room, using low wall exhaust grills and dampers. Dampers are another blog for another day because balancing is critical at 0.010” wc to 0.030” wc. Balancing the C-SEC’s cubic box with low wall exhausts is easier than balancing the box with one, two, or many C-PECs connected externally.
The information here may still seem a bit cloud, muddy, or just unclear and I certainly understand that if that is your takeaway. When you live inside of a specific topic-bubble (just made that up) it is easy to lose sight of the fact that others do not have the repetitive and intimate knowledge of the topic. If this is the case for you after reading this BLOG post, please feel free to email me firstname.lastname@example.org or call me direct at 228-239-6842 and let’s have a more in-depth discussion on this topic. Thanks for reading!