Castro Valley pollen forecast - today, tomorrow and the week ahead
Pollen Today
Today (20 September) pollen in Castro Valley is high; ragweed is high; grass is moderate.
Pollen Tomorrow
Tomorrow (21 September 2026) grass remains moderate, ragweed remains high, mugwort at low levels.
7-Day Trend
Over the next week mugwort will increase; grass and ragweed will decrease.
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Pollen Risk
Today - 20 September
| Pollen species | Level | Value |
|---|---|---|
| Alder (Alnus) | None | 0/100 |
| Birch (Betula) | None | 0/100 |
| Hazel (Corylus) | None | 0/100 |
| Cypress (Cupressaceae) | None | 0/100 |
| Grass (Poaceae) | Moderate | 29/100 |
| Willow (Salicaceae) | None | 0/100 |
| Mugwort (Artemisia) | None | 0/100 |
| Ragweed (Ambrosia) | High | 58/100 |
Tomorrow - 21 September
| Pollen species | Level | Value | 7-Day Trend |
|---|---|---|---|
| Alder (Alnus) | None | 0/100 | — |
| Birch (Betula) | None | 0/100 | — |
| Hazel (Corylus) | None | 0/100 | — |
| Cypress (Cupressaceae) | None | 0/100 | — |
| Grass (Poaceae) | Moderate | 43/100 | |
| Willow (Salicaceae) | None | 0/100 | — |
| Mugwort (Artemisia) | Low | 8/100 | |
| Ragweed (Ambrosia) | High | 70/100 |
Local Environmental Conditions
Castro Valley's East Bay California setting features blue oak, California bay, and eucalyptus as the main tree pollen sources, with exposure varying between the extensively wooded chaparral hillsides of the Castro Valley hills and the more built-up commercial corridors along Castro Valley Boulevard. The elevation gradient between the oak and bay woodland above and the valley floor below creates distinct local pollen zones throughout the spring season.
Castro Valley's East Bay California setting produces a season opening in January with oak and olive before grass from surrounding Alameda County land peaks through April and May. Mugwort extends exposure into September, while the bay proximity moderates temperatures and the East Bay hills channel the Delta breeze that can periodically suppress or intensify pollen dispersal throughout the spring season.
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Frequently Asked Questions About Pollen in California
Is pollen high in Castro Valley right now?
On 20 September 2026, overall pollen levels in Castro Valley are high. Ambrosia is the main contributor to airborne pollen, while grass remain at lower levels
Will pollen levels change in Castro Valley tomorrow?
On 21 September 2026, pollen conditions in Castro Valley will be different from today, with ambrosia at high levels, artemisia at low levels, grass at moderate levels
Is pollen expected to decrease in Castro Valley this week?
According to the five-day outlook, ambrosia and grass pollen levels are expected to gradually decrease, while artemisia pollen will increase, suggesting mixed conditions later in the week.
Which pollen types are currently active in Castro Valley?
On 20 September 2026, Ambrosia is the dominant pollen source in Castro Valley. grass are also present but at lower levels. artemisia are not currently active.
When is pollen season in Castro Valley?
How often is the Castro Valley pollen forecast updated?
The Castro Valley pollen forecast is updated four times daily. See the timestamp below for the most recent update.
Does pollen in Castro Valley come from nearby or far away?
Studies show that most pollen exposure (often 70–90%) comes from nearby sources, such as local trees and vegetation. A good pollen forecast therefore depends on accurately representing local vegetation. Pollen can travel long distances, but this usually matters only early in the season or during specific wind events.
Are pollen levels in Castro Valley based on measurements or forecasts?
Pollen data is often misunderstood. Most "pollen levels" are not real-time measurements, but forecasts.
Direct pollen measurements are typically delayed by 1-2 days because they require manual analysis. To show current conditions in Castro Valley, modern forecasts combine recent measurements, weather data, and information about local vegetation.
This allows us to estimate pollen levels for today and the coming days, not just report the past.
Airmine’s atmospheric monitoring platform was developed with support from the European Space Agency (ESA)’s Business Applications and Space Solutions (BASS) team, using satellite and Earth observation technologies to improve emissions monitoring.
Understanding the Scale
The 0–100 scores represent risk levels, not direct measurements of pollen or air pollutant concentrations. Higher scores indicate higher risk. Pollen and air quality use different thresholds.
Pollen
| None | 0 | No or negligible risk of exposure. Pollen/mold exposure is expected to be minimal. |
| Low | 0-25 | Low risk of exposure. Small amounts of pollen or mold spores may be present. |
| Moderate | 25-50 | Moderate risk of exposure. Pollen/mold exposure is expected to be noticeable. |
| High | 50-75 | High risk of exposure. Elevated pollen/mold exposure is expected. |
| Very High | 75-100 | Very high risk of exposure. Pollen/mold exposure is expected to be at or near its highest levels. |
Get More in the Airmine Pollen app
Get 15-day pollen forecasts with morning, afternoon and evening detail, plus 5-day air quality forecasts. Explore interactive pollen risk maps, estimated pollen season start, route profiles and a symptoms diary.
Features and forecast availability vary by region. Currently covering Europe, the contiguous US, southern Canada, South Africa, Australia, Indonesia (Java & Bali), Brazil, Paraguay, Uruguay and the Buenos Aires area.
Developed with support from the European Space Agency (ESA) Business Applications and Space Solutions (BASS).
