Typical air vs. spike days
- Annual average PM2.5 (9.14 µg/m³) reflects usual daily exposure.
- Worst-day peak PM2.5 (120.80 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 Snohomish without contradicting EPA-aligned thresholds.
Pollen and mold from the Snohomish River valley create a heavy seasonal load on HVAC systems.
The local geography, including proximity to the Centennial Trail and the river, means high concentrations of tree pollen in the spring and mold spores during the damp autumn months. These biological contaminants act as a 'pre-filter' load, filling up the surface area of your air filter and reducing its ability to catch finer PM2.5 particles during air quality spikes. Residents often notice a visible buildup of dust and organic material on their filters during the transition between the rainy season and the dry summer months.
Because PM2.5 peaks exceed 120 µg/m³, we recommend a MERV 13 filter for all Snohomish homes.
A standard MERV 8 filter is insufficient for the extreme spikes recorded in this area. A MERV 13 filter is designed to capture the fine particulates that characterize the city's worst-day events. However, these high-efficiency filters have denser pleats and can restrict airflow if they become too dirty. Given the local humidity and seasonal pollen, you must change these filters every 60 to 90 days to protect your blower motor. If you live near heavily wooded areas or the river, check the filter monthly. For maximum protection during the recorded 120.8 µg/m³ events, ensure your HVAC fan is set to 'On' rather than 'Auto' to keep air moving through the filter continuously until the outdoor air clears.