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High humidity levels during the summer months can cause a number of challenges for building owners and occupants, for a few months of each year mold is often one of those challenges. While 30 – 50% relative humidity (RH) is ideal, we don’t usually see mold growth until indoor relative humidity exceeds 60%. On a hot day (think 90 degrees and 90% humidity) it’s common to exceed 60% RH indoors for significant periods of time, especially if units are over or undersized, or if moisture is being introduced into the space. Although A/C units dehumidify as they run they are not dehumidifiers, in fact most HVAC systems are specifically designed to meet temperature requirements only. Specialized systems for data centers, laser printing, pharmaceutical manufacturing and other processes are available but come with a price tag (both upfront and operating costs) that tend to be prohibitive. Another possible strategy is to use particulate removal through filtration, for example using HEPA filters (which remove 99.97% of particulates greater than .3 microns) and simply remove the mold spores before they can grow, however the cost to install and operate these systems are very high which is why they are usually reserved for clean rooms and surgical suites.


There are thousands of mold species and they can grow on virtually any organic material as long as moisture and oxygen are present. Many molds pose health risks to building occupants, and long term mold growth will physically damage whatever it is growing on. Eliminating mold and mold spores indoors is virtually impossible, therefore controlling indoor moisture tends to be the most effective strategy. In order to prevent mold growth make sure housekeeping is in order, HVAC and sump pump systems are properly maintained, and building envelopes are water tight (e.g. roofs, foundations, and windows). Keep an eye out for any moisture signs where it doesn’t belong, the sooner problem areas are cleaned and dried out the better. Sometimes an industrial hygienist is needed to help determine what is causing a mold problem in the first place, and once the underlying problem is solved using a remediation company is often the most effective way to remedy (dry, remove, repair) a situation quickly.


For our part we try to keep all equipment clean and fully charged so it can operate efficiently and effectively, and to minimize potential problems inspecting drain pans and checking for dirty evaporator coils and/or water leaks are part of our normal preventive maintenance tasking. In order to meet fresh air code requirements many systems are designed to bring in more fresh air than they could possibly dehumidify. The old saying in the industry is that “the solution to pollution is dilution” which put a lot of emphasis in bringing in large amounts of outdoor air. This works to a degree but in an air conditioned building it can unintentionally cause problems by actually increasing relative humidity. Thankfully there are ways to meet code requirements and effectively manage RH by using a combination of demand ventilation, dehumidification, and energy recovery units. These solutions are most cost effective when part of a building’s original design, however these strategies can also be applied to existing buildings when problems arise.


We may not be able to control what or how many mold spores are in the air at any given time, and we certainly can’t control the weather… but through good housekeeping , good design, and regular maintenance we can minimize the chances of mold problems in our buildings and protect our people from the potential health risks that accompany them.


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