Typical air vs. spike days
- Annual average PM2.5 (8.22 µg/m³) reflects usual daily exposure.
- Worst-day peak PM2.5 (39.16 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 San Rafael without contradicting EPA-aligned thresholds.
Local Allergen Load
San Rafael's proximity to the the city Hills and various open spaces means that seasonal pollen is a major factor for indoor air quality. Oak and pine pollen are heavy contributors in the spring, while late-summer grasses add to the particulate load. This biological material is often larger than PM2.5 but is equally effective at clogging HVAC filters and reducing system efficiency. In the more humid months, mold spores from the bay and local canyons can also become an issue. These allergens create a constant demand on your air filter, making it necessary to monitor the physical condition of the filter rather than just following a calendar schedule.
HVAC Filter Recommendations
For the city homes, a MERV 11 filter is the minimum baseline, but a MERV 13 is highly recommended to handle the 39.16 µg/m³ PM2.5 peaks. MERV 13 filters are much more effective at capturing the fine particulates that characterize these spikes. If your system is older and has limited airflow, a MERV 11 paired with a standalone HEPA filter in high-traffic rooms is a better setup than forcing a thick filter into a small slot. Given the ozone peaks near 0.060 ppm, a filter with an integrated activated carbon layer can help strip out odors and gaseous irritants. Change your filters every 90 days under normal conditions, but drop that to 60 days during peak pollen seasons or if you live near high-traffic corridors where dust levels are naturally higher. Keeping a fresh filter ensures your furnace doesn't overheat while trying to pull air through a clogged medium.