Flies might be an annoying nuisance when you are trying to enjoy a mocha at your local coffee shop, but a health inspector will use terms such as “vector of disease” and “food safety risk” to describe these tiny, winged insects. Whether you operate out of a storefront or a stand-alone food production facility, these little pests can be a big issue, regardless of the auditor or inspector.

But what happens when controlling these pests seems impossible? For instance, even though you have excellent sanitation, a well-maintained exterior envelope and you may be performing strong pest management techniques, uncontrolled arthropods may be a persistent problem in your facility. Before they drive you mad, consider if the facility simply sucks.

Let me explain! I am not suggesting that your facility is necessarily bad. I am suggesting the building’s air pressure may be negative, which means it sucks air, and arthropods, in. That is why you must always consider negative air pressure (NP) in your pest control solutions.

What is negative air pressure?

NP is the force that pulls air into a building like a giant facility-wide vacuum. Small arthropods that are naturally impacted by air movement (e.g., midges, mosquitoes, spiders that use ballooning, etc.), as well as some that are not, may be forced inside. This can cause thousands of pests to unwillingly enter a single window each day and may render traditional control measures ineffective. It does not matter if the problem is in your food processing facility or your warehouse — NP presents a significant food safety issue.

The most obvious pest management symptom of NP is significantly elevated flying insect activity inside. NP is powerful enough to draw midges straight through a window screen, which can in turn drive a facility to extreme preventative measures. That is why, when I discover a facility is installing breezeways at every entrance, covering window openings with furnace filter media and requesting frequent exterior pesticide treatments, I understand all of these to be secondary symptoms of a NP problem. It is remarkable to see that level of dedication to pest management prevention. However, those corrective actions usually do not solve the pest problem.

Physical signs of negative air pressure.

Too many flies in your facility? Maybe your building sucks.
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The most significant non-pest related sign that points to a NP issue is difficulty entering an exterior door.

When extra force is required to pull an exterior door open an inch before the door becomes free and swings open normally, you may have a NP problem. You also may see an adjacent breezeway door magically open or ceiling tiles flutter when you open one exterior door.

If the problem is not so evident, performing the tissue test might reveal a less forceful NP issue. While your facility’s production lines and exhaust fans are running, place a tissue against the outside of a production-area window screen. If the tissue sticks to the screen, then a NP problem exists. I will explain the production importance later.

Causes of negative air pressure.

When a building is being constructed, its air pressure is neutral, just like outside air, and freely moves inside and outside. Once the windows, doors and ventilation system are installed, the building is placed under pressure. If more force is directing air into the building than out, it has positive pressure. The opposite is also true. If more air is directed out than in, the building has negative pressure. When that happens, negative pressure makes up the difference by forcing air (and pests) in through windows, doors and gaps.

When a commercial/industrial facility is built, air pressure needs are assessed, and a HVAC system is designed to introduce enough treated fresh air to keep air pressure as close to neutral as possible without becoming negative. This treated, or filtered, air is called make-up air. Considerations for needed make-up air include estimating how much ventilation the facility will need and how much air will be exhausted from equipment. For instance, if a piece of food-processing equipment will exhaust through a roof vent, the facility needs to make up the difference to keep air pressure in check.

Your building’s air pressure may be negative, which means it sucks air, and arthropods, in.
© LariBat | Adobe Stock

You may notice NP issues arise when equipment or ventilation is added. During the excitement of growth, the process of quickly installing new equipment, as well as budgetary pressures, may mean not conducting a formal building pressure diagnosis, and make-up air might be neglected. This is the typical reason for NP issues. Though it seems like a harmless oversight, the extra exhausted air creates pressure without notice and that pressure void demands outside air. Immediately, nearby gnats, who are enjoying their brief life outside, are vacuumed into the interior of the facility.

How to turn a negative into a positive.

Navigate the challenges of NP by following these three steps:

  1. Identify the presence of NP by performing the tissue test during production. From outside the facility, place a tissue against the outside of a production-area window screen. Be certain the facility is in full operation during the test. If all exhausting and venting equipment is not operating, the building will essentially be depressurized and not give accurate results.
  2. If discovered, educate your executive team on the facts of negative air pressure. Teach them what NP is, how it becomes a problem and its evidence in the facility. Explain small arthropod susceptibility to NP and connect it to the presence of arthropods in the facility.
  3. Hire a HVAC contractor to address the building’s NP. They may perform a formal building pressure diagnosis. They can design and install a make-up air system.

If the thought of expending significant capital for a make-up air unit seems to be too much for your company, consider what one facility we serve did. They had a temporary make-up air system installed in a seldom-used receiving dock. This allowed them to see the impact of a new system firsthand. After a season with the new system in place and roughly a 95% decline in flying insect activity, they were able to justify the installation of a permanent make-up air system.

So, if flies are driving you crazy, keep your chin up and keep it positive (air pressure, especially). Positive pressure may be just what you need to keep your facilities and food safe.