Typical air vs. spike days
- Annual average PM2.5 (9.27 µg/m³) reflects usual daily exposure.
- Worst-day peak PM2.5 (34.66 µg/m³) is what filtration must handle during bad-air events.
Sections below reference one or both metrics on purpose — that is how HVAC vs. portable guidance differs for Ann Arbor without contradicting EPA-aligned thresholds.
Seasonal Pollen and Mold Loads
Seasonal shifts in the Huron River valley bring heavy loads of tree and grass pollen, followed by mold spores in the humid late summer. These biological particles settle in the ductwork and on the cooling coils of your AC unit. In this region, the transition from heating to cooling seasons often kicks up accumulated dust. Even if the outdoor air looks clear, the indoor dust load increases as the HVAC system cycles more frequently. A high-quality filter acts as the primary defense against these seasonal irritants, preventing them from being redistributed into living spaces during peak pollen counts.
Technician's Filter Recommendations
Based on a maximum PM2.5 of 34.66 µg/m³, I recommend a MERV 13 pleated filter. Standard MERV 8 filters are designed to protect the equipment, not your lungs, and they won't catch the fine particles seen during local spikes. Because ozone peaks reach 0.0721 ppm, a filter with an activated carbon layer is highly effective at neutralizing odors and gaseous pollutants that bypass standard media. In the local climate, humidity can lead to moisture buildup on filters, which traps more dust but can also restrict airflow if left too long. Change your MERV 13 filter every 60 to 90 days. If you have pets or high foot traffic, 60 days is the hard limit to prevent the blower motor from straining against a clogged filter.