SmogRank / United States / Colorado / Boulder

Boulder, month by month

Boulder has a measured daily mean on 993 days since 1 January 2023. Its worst month was May 2023, at 53 US AQI (10.1 µg/m³), 455th of 1,600 cities on that month's world ranking. Its cleanest was February 2026, at 16 US AQI (2.8 µg/m³).

Every month since 2023

202627 so far277 days measured2,664th of 2,791 in the world
Jan18#2,560Feb16#2,829Mar18#2,903Apr22#2,738May19#2,760Jun36#1,735Jul45#1,848Aug51#1,132Sep18#2,889Oct20#2,576
202529 for the year359 days measured2,488th of 2,681 in the world
Jan24#2,331Feb38#1,889Mar17#2,788Apr26#2,650May23#2,545Jun34#2,145Jul43#1,850Aug47#1,674Sep36#1,933Oct22#2,390Nov24#2,531Dec18#2,930
2024too few days for a mean (a year needs 257)196 days measured
JannoneFeb1 dayMar1 dayAprnoneMaynoneJun18 daysJul46#1,233Aug33#1,752Sep32#1,594Oct44#830Nov22#2,458Dec17#2,552
2023too few days for a mean (a year needs 256)161 days measured
Jan42#768Feb1 dayMar10 daysApr26#1,393May53#455Jun31#1,370Jul39#972Aug15 daysSepnoneOctnoneNovnoneDecnone
Each square is a month's mean PM2.5 in US AQI from Boulder's measured days, with its place on that month's world ranking under it. A month has a mean once 70% of its days are measured, the rule the monthly rankings use; under that, the square gives the days measured. Each square opens that month's ranking with Boulder marked; each year, the year's. A dashed square is the running month, so far.
Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales

Months without a mean, and why

13 months
MonthWhy
Jun 202418 of 30 days measured; a month needs 21. The others: 12 with under 18 hours of readings or readings that failed our checks.
May 2024No day measured. The others: 31 with under 18 hours of readings or readings that failed our checks.
Apr 2024No day measured. The others: 30 with under 18 hours of readings or readings that failed our checks.
Mar 20241 of 31 days measured; a month needs 22. The others: 30 with under 18 hours of readings or readings that failed our checks.
Feb 20241 of 29 days measured; a month needs 21. The others: 28 with under 18 hours of readings or readings that failed our checks.
Jan 2024No day measured. The others: 31 with under 18 hours of readings or readings that failed our checks.
Dec 2023No day measured. The others: 31 with under 18 hours of readings or readings that failed our checks.
Nov 2023No day measured. The others: 30 with under 18 hours of readings or readings that failed our checks.
Oct 2023No day measured. The others: 31 with under 18 hours of readings or readings that failed our checks.
Sep 2023No day measured. The others: 30 with under 18 hours of readings or readings that failed our checks.
Aug 202315 of 31 days measured; a month needs 22. The others: 16 with under 18 hours of readings or readings that failed our checks.
Mar 202310 of 31 days measured; a month needs 22. The others: 20 missing from OpenAQ's archive, 1 with under 18 hours of readings or readings that failed our checks.
Feb 20231 of 28 days measured; a month needs 20. The others: 25 missing from OpenAQ's archive, 2 with under 18 hours of readings or readings that failed our checks.

A day counts once 18 hours of it are measured; a month needs 70% of its days.

Boulder's worst months

MonthUS AQIDays measuredWorld rankEpisode
May 2023 5310.1 µg/m³28 of 31455thof 1,600North-western United States and Canada, May 2023
August 2026 519.2 µg/m³31 of 311,132ndof 2,896North-western United States and Canada, July and August 2026
August 2025 478.4 µg/m³31 of 311,674thof 2,901–
July 2024 468.3 µg/m³31 of 311,233rdof 2,566–
July 2026 458.1 µg/m³30 of 311,848thof 2,784North-western United States and Canada, July and August 2026
October 2024 447.9 µg/m³29 of 31830thof 2,020–

Monthly means over the days measured; finished months only. World rank: its place on that month's world ranking of every city with enough days. Each month opens that ranking.

Boulder's worst days

DayUS AQIEpisode
19 May 2023 16169.6 µg/m³North-western United States and Canada, May 2023
20 May 2023 10537.1 µg/m³North-western United States and Canada, May 2023
3 Aug 2026 9432.3 µg/m³North-western United States and Canada, July and August 2026
1 Aug 2024 9029.9 µg/m³–
4 Aug 2026 8527.5 µg/m³North-western United States and Canada, July and August 2026
28 Jun 2026 8225.7 µg/m³–
4 Sept 2025 8125.0 µg/m³Colorado and Wyoming, United States, September 2025
7 Aug 2026 7823.5 µg/m³–
22 May 2023 7823.4 µg/m³North-western United States and Canada, May 2023
5 Sept 2025 7622.4 µg/m³–

Daily means, the highest since 1 Jan 2023; the WHO's daily guideline is 15 µg/m³. Each date opens that day's world ranking, and each episode the bad-air spell it was part of.

Use this history

SmogRank, PM2.5 monthly and daily means, Boulder, United States, 1 Jan 2023 – 11 Oct 2026. Monitor data from AirNow, via OpenAQ and from the publisher directly; sources listed at smogrank.com/sources. smogrank.com/us/boulder/history (accessed 12 Oct 2026).

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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.

Boulder now: the last 24 hours, its ranks today and the last 12 months day by day.

Updated 2026-10-12 17:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index. Some low-cost readings: Sensor.Community, ODbL (data and licence). Some readings: Uzhydromet (Uzbekistan's Hydrometeorological Service); Department of Environment, Nepal; Vietnam's government monitors; Aurassure; Umhverfisstofnun (Environment Agency of Iceland); RMCAB (Secretaría Distrital de Ambiente, Bogotá); Love My Air (City and County of Denver), no reuse terms published (why we use them).