Prairie soil, field dust and seasonal air
The ground east of Austin is often dark clay from the blackland prairie. In dry weather it cracks and turns to powder, and wind lifts it from plowed fields and graded lots. Mowing, haying and burning off fields can add particles, and spring pollen from grasses and oaks follows the winter cedar season further west.
For a duct system, that means fine dust loads both filters and coils. Homes near open land may see filters turn dark within weeks. Rural homes are also more likely to collect insects, mud dauber nests and rodent debris around outdoor units and vent caps, which is worth checking before trouble starts.

