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San Francisco's mild, maritime climate produces a relatively long but moderate allergy season, with cool, wet winters and dry summers shaping a year-round exposure pattern rather than sharp seasonal spikes.
Tree pollen is typically the first major trigger, emerging as early as January and peaking from February through April. Common culprits include oak, cypress, juniper, olive, walnut, alder, and the widely planted acacia and eucalyptus, both of which are notorious for producing heavy, irritating pollen loads across the Bay Area.
As trees taper off, grass pollen takes over from late April through July, dominated by Bermuda, rye, fescue, and Kentucky bluegrass, with windy afternoons often carrying pollen in from surrounding hillsides. The tail end of grass season frequently overlaps with early weed pollination, intensifying symptoms in late spring.
Weed pollen peaks from August through October, driven by ragweed (less severe here than inland), sagebrush, pigweed, lamb's quarters, and plantain.
Non-pollen allergens are also significant: coastal fog and damp housing stock encourage indoor mold, dust mites thrive in the humid microclimate, and traffic-related air pollution can amplify respiratory irritation.
Overall, San Francisco's allergy profile is defined by a prolonged, overlapping season of moderate pollen levels compounded by persistent mold and urban air quality issues.