EnviroNearby

Lassen County, California air quality history

Daily Air Quality Index summaries from EPA monitors in Lassen, California, 1996–2001.

88%
Good days in 2001
29
Median daily AQI, 2001
0
Days above 100 in 2001
40
Days with an AQI in 2001 (of 365)

In 2001, EPA reported a daily Air Quality Index (AQI) for Lassen, California on 40 days. 35 of those days (88%) were in the Good range, and the median daily AQI was 29. That median is lower than the typical monitored county in California (median 44.5 across 52 counties), and lower than the typical monitored U.S. county (44). The main pollutant was coarse particles (PM10) on 40 days.

EPA did not report an AQI for every day of 2001 here (40 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

The most recent year with data for this county is 2001; EPA's files for 2002–2025 contain no row for it, which usually means its monitors stopped reporting.

Years with fewer than 120 monitored days (1996, 1997, 1998, 1999, 2000, 2001) are shown in the table but left out of comparisons and trend statements.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1996: Good 10 of 10 days19961997: Good 36 of 38 days1997: Moderate 2 of 38 days19971998: Good 14 of 14 days19981999: Good 46 of 53 days1999: Moderate 7 of 53 days19992000: Good 26 of 33 days2000: Moderate 7 of 33 days20002001: Good 35 of 40 days2001: Moderate 5 of 40 days2001
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001996: 90th percentile AQI 391997: 90th percentile AQI 461998: 90th percentile AQI 481999: 90th percentile AQI 542000: 90th percentile AQI 542001: 90th percentile AQI 551996: Median AQI 181997: Median AQI 221998: Median AQI 321999: Median AQI 282000: Median AQI 282001: Median AQI 29199619971998199920002001
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2001

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Coarse particles (PM10)40100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2001403550000295576PM10
2000332670000285463PM10
1999534670000285473PM10
1998141400000324848PM10
1997383620000224665PM10
1996101000000183939PM10

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

All monitored counties in California

Source: U.S. EPA AirData, “Annual AQI by County” files for 1996–2001 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem