Gross, Idaho

Pollen is rated on the National Allergy Bureau bands for the grains of pollen in a cubic meter of air. Trees: Low under 15, Moderate 15 to 89, High 90 to 1,499. Weeds: Low under 10, Moderate 10 to 49, High 50 to 499. Grass: Low under 5, Moderate 5 to 19, High 20 to 199. The band is the one reached on the busiest quarter of counted days. A rating is typical, not a forecast: one wet or windy year can move it.

Spring trees, Moderate. Boise NAB station, 62 counted days in 2017 to 2021: on the busiest quarter of days the tree count was 74 grains per cubic meter or more, which the NAB bands call Moderate. On the busiest tenth of days it was 104 or more. That is a short record, so treat it as a rough guide. The Allergy Group's own Boise counts agree: 25 to 30 tree pollen on a daily sample starts mild to moderate symptoms, and cottonwood and juniper reach high counts in April in dry springs.

Early-summer grass, High. The Allergy Group (Boise) reports grass counts over 100 on its 24-hour sampler in early June, and 30 is enough to bother people who react to grass. NAB calls 20 to 199 High. The NAB grass bands: Low under 5, Moderate 5 to 19, High 20 to 199, Very High 200 or more grains per cubic meter.

Fall weeds and sagebrush, High. Boise NAB station, 37 counted days in 2017 to 2019: on the busiest quarter of days the weed count was 54 grains per cubic meter or more, which the NAB bands call High. On the busiest tenth of days it was 73 or more. That is a short record, so treat it as a rough guide. The Allergy Group's own Boise counts agree: chenopod (tumbleweed, kochia, Russian thistle) peaks at 30 to 60 in late summer, and sagebrush runs 400 to 500 on a good day from mid-September to mid-October.

Wildfire smoke, High. The average number of days a year the EPA's air quality index passed 100 ("unhealthy for sensitive groups" or worse), 2021-2025. Under 3 days is Low, 3 to 9 Moderate, 10 or more High. That grouping is ours, not the EPA's. Gem County has no EPA monitor, so this uses Boise County, 20 miles away. The count includes any cause, but across Idaho's monitors 86% of those days fell in July to September and 94% were fine particles (PM2.5), which is what wildfire smoke is made of (570 county-days, 2021-2025).

Year to year. The Asthma and Allergy Foundation of America ranked Boise 95th of 100 U.S. metros for pollen in its 2025 report and 1st in its 2026 report, after a wet winter and a long grass season. Expect a typical rating to miss some years.

Not rated. Mold: no published mold spore counts for Idaho towns are in the data used, so there is no mold rating. House dust, pets and food allergies are outside this rating.