Bad Air Days in Guaynabo, Puerto Rico: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 550 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (44% particle days).

When the bad days come in Guaynabo

Most bad days here fall in January and June. Averaged over 2016 to 2025.

Is a purifier worth it here?

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The record for Guaynabo

In 2000 to 2009 this county averaged under 1 bad days a year; in 2016 to 2025, 2. Not much has changed. The worst recent year was 2019, with 4 bad days on 362 monitored days.

Date	AQI	Main pollutant
2018-06-19	176	PM2.5
2017-05-22	151	PM2.5
2016-07-06	119	NO2
2018-09-20	116	PM2.5
2019-08-29	110	NO2

Most days, not the worst ones

Across 1,909 monitored days from 2016 to 2025, the air in Guaynabo rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 13%. The median day sat at AQI 20; one day in ten went above 52.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)48%924 days
nitrogen dioxide37%703 days
carbon monoxide11%212 days
coarse dust (PM10)4%70 days

Guaynabo by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	114	93	21	0	0	33	67
2024	115	91	24	0	0	31	81
2023	107	82	25	0	0	32	74
2022	94	81	13	0	0	29	80
2021	119	86	33	0	0	33	77
2020	83	68	15	0	0	29	75
2019	362	312	46	4	0	22	110
2018	302	281	18	3	1	10	176
2017	248	232	15	1	1	11	151
2016	365	328	36	1	0	14	119
2015	353	298	55	0	0	27	100
2014	329	295	33	1	1	25	253
2013	331	292	39	0	0	29	68
2012	330	273	57	0	0	30	83
2011	336	305	31	0	0	26	65
2010	347	315	32	0	0	26	100

*2025 may be a partial year in EPA's file.

Puerto Rico compared

Catano11/yr
Juncos5/yr
Bayamon3/yr
Mayagnez3/yr
Ponce2/yr
Caguas1/yr
Fajardothin data
Adjuntasthin data

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.