Delaware Air Quality: The Seasonal Shape

Q: Which months have the worst air quality in Delaware?

In EPA's monitoring record for 2020 to 2024, the worst stretch in Delaware is May through July. June leads: 5.1% of monitored days reached Unhealthy for Sensitive Groups or worse and 43% were at least Moderate, with the index most often set by ozone. July runs second. The month-by-month chart on this page shows the full shape.

Q: Does wildfire smoke affect Delaware?

Yes, episodically. Delaware's ordinary calendar is summer ozone season, but smoke transport can override it in a bad continental fire year; the worst single day in the five year record was AQI 282 on June 7, 2023. Smoke days arrive with the wind, not the calendar, which is exactly what the live NWS alert feed on this page is for.

Q: What is the cleanest month for air quality in Delaware?

September, in the 2020 to 2024 record: only 17% of monitored days even reached Moderate, and 0.0% went beyond it. If you are choosing a month for a visit or an outdoor project on air quality grounds, that is the statistical answer.

Q: Where do I check the live AQI for Delaware today?

AirNow.gov, EPA's official public AQI service, publishes the current reading and forecast at its Delaware state page. This page adds the calendar (which months are like this every year, and why) plus the live NWS alert products above, reproduced as issued.

Q: How is this monthly calendar computed?

From EPA's Air Quality System pre-generated daily AQI files, the official regulatory record: every daily AQI value for every monitored Delaware county, 2020 to 2024, grouped by calendar month into EPA's six categories. The snapshot is rebuilt annually when EPA finalizes a new year. It is climatology, not a forecast: it says what the months are usually like, not what next week holds.

Summer ozone season. Which months are worst in Delaware, why, and what the EPA record actually shows.

Worst stretch in the EPA record: May through July

Worst month

June

Cleanest month

September

Season type

Summer ozone season

Delaware air quality at a glance

EPA record, 2020 to 2024

June

worst month

5.1% of days USG or worse

ozone

sets the index then

statewide, in June

AQI 282

worst single day

June 7, 2023

September

cleanest month

17% of days even reach Moderate

Computed from EPA's official daily AQI record for every monitored county in Delaware, 2020 to 2024. Shares are of monitored county days, so they describe how often air somewhere in the state crossed each line.

The calendar

Which months are worst for air quality in Delaware

The chart below is five years of EPA's regulatory monitoring record, 2020 to 2024, folded into a single year: for each month, the share of monitored days in each official AQI category, in EPA's exact colors. Under each bar is the pollutant that most often set the index that month, which is the mechanism in one word: ozone is a sunshine chemistry problem, PM2.5 in winter is a trapped-air problem, PM2.5 in late summer is usually smoke.

JanPM2.5

FebPM2.5

MarO₃

AprO₃

MayO₃

JunO₃

JulO₃

AugPM2.5

SepO₃

OctO₃

NovPM2.5

DecPM2.5

Share of monitored days in each EPA AQI category by month, all Delaware counties with monitors, 2020 to 2024. Source: EPA Air Quality System daily AQI files.

The three worst months, ranked

June

5.1% of monitored days reached Unhealthy for Sensitive Groups or worse, and 43% at least Moderate. The index was most often set by ozone. Worst single day: AQI 282 on June 7, 2023.

July

1.3% of monitored days reached Unhealthy for Sensitive Groups or worse, and 52% at least Moderate. The index was most often set by ozone. Worst single day: AQI 144 on July 20, 2021.

May

0.6% of monitored days reached Unhealthy for Sensitive Groups or worse, and 28% at least Moderate. The index was most often set by ozone. Worst single day: AQI 115 on May 24, 2024.

Ranked by share of days reaching Unhealthy for Sensitive Groups or worse, the first category EPA ties to health effects for anyone. A single bad fire year inside the window can move a month; that is not noise, that is what living with a smoke era means.

The mechanism

Why Delaware air goes bad when it does

Delaware's calendar is the mid-Atlantic standard: ozone in the hot months, peaking in July, with the northern industrial corridor around Wilmington reading worse than the beaches. The June 2023 smoke event supplied the worst particulate days in its recent record.

Ozone transported up the I-95 corridor plus the state's own refinery and traffic emissions.
Hot stagnant Chesapeake and Delaware Bay summers that let the plume build for days.

The data shows the handoff plainly: in midsummer the index in Delaware is most often set by ozone, while in the depths of winter it is fine particles (PM2.5). Two different problems wearing one scale.

An airport control tower barely visible through dense orange wildfire smoke at midday during the June 2023 Canadian smoke event over the New York area — Midday, June 7, 2023, in the New York City area: Canadian wildfire smoke thick enough to hide an airport control tower. That event reset expectations across the East, Delaware included, about what a smoke day can look like a thousand miles from the fire. “Quebec Canada Wildfire Smoke Consumes New Jersey and New York City June 7 2023” by Anthony Quintano, CC BY 2.0

Live, as issued

Air quality alerts in Delaware right now

These are the active National Weather Service air quality products whose zones touch Delaware, reproduced as issued with the issuing office attached. An Air Quality Alert is written by or for your state's air agency; NWS also issues its own smoke, dust and stagnation advisories. Each is reproduced in full.

Checking api.weather.gov for active air quality alert products…

Today's AQI is at AirNow

EPA's AirNow page for Delaware publishes the current reading and the official forecast, fed by the same agencies whose alerts appear above. This page carries the other half of the picture: which months are like this every year, and why.

EPA air sampling instruments on tripods inside a fenced monitoring site, with a technician in an EPA contractor shirt walking past — EPA air sampling stations at a monitoring site. Every number on this page traces back to instruments like these: the monthly chart is computed from EPA's official daily AQI record for every monitored Delaware county, 2020 to 2024. “EPA Air Sampling Stations” — US EPA — Public domain

Reading the index

The AQI scale, in EPA's words

Every category on this page uses EPA's official names, ranges and colors, quoted rather than paraphrased. The line that matters most for planning is 101: Unhealthy for Sensitive Groups, the first category EPA ties to health effects, and the threshold the worst-month ranking above counts.

AQI	Category	What it means
0 to 50	Good	Air quality is satisfactory, and air pollution poses little or no risk.
51 to 100	Moderate	Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
101 to 150	Unhealthy for Sensitive Groups	Members of sensitive groups may experience health effects. The general public is less likely to be affected.
151 to 200	Unhealthy	Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
201 to 300	Very Unhealthy	Health alert: The risk of health effects is increased for everyone.
301 and higher	Hazardous	Health warning of emergency conditions: everyone is more likely to be affected.

Category names, index ranges and colors are reproduced exactly from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Tables 1 and 2. Meanings quoted from AirNow.gov, AQI Basics.

Who EPA means by sensitive groups

When this pollutant has an AQI above 100	EPA lists these sensitive groups
Ozone	People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults.
Fine particle pollution (PM2.5) and particle pollution (PM10)	People with heart or lung disease; older adults; and children, including teenagers. In addition, some minority groups, people with lower incomes, and outdoor workers may experience higher exposure that can worsen underlying health conditions.
Carbon monoxide (CO)	People with heart disease.
Nitrogen dioxide (NO2)	People with lung disease, such as asthma; children, including teenagers; and older adults.
Sulfur dioxide (SO2)	People with asthma; children, including teenagers; and older adults.

What EPA says sensitive groups should do on an orange day

Sensitive groups: Make outdoor activities shorter and less intense. Take more breaks. Watch for symptoms such as coughing or shortness of breath. Plan outdoor activities in the morning when ozone is lower. People with asthma: Follow your asthma action plan and keep quick-relief medicine handy.
EPA cautionary statement for Ozone (8-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Sensitive groups: Make outdoor activities shorter and less intense. It's OK to be active outdoors but take more breaks. Watch for symptoms such as coughing or shortness of breath. People with asthma: Follow your asthma action plan and keep quick relief medicine handy. People with heart disease: Symptoms such as palpitations, shortness of breath, or unusual fatigue may indicate a serious problem. If you have any of these, contact your health care provider.
EPA cautionary statement for PM2.5 (24-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Everything above is EPA’s language, quoted. On any day that matters, the decision belongs with the official product for your location and your own clinician.

Downtown Seattle skyline nearly erased by dense wildfire smoke, with traffic moving on Interstate 5 in the foreground — Downtown Seattle disappearing into wildfire smoke on September 11, 2020. Even states far from the fire belt now plan around weeks like this: smoke rides the continental wind, and the calendar question is when your corridor opens. “Smokey skyline view of Downtown Seattle from Rizal Bridge - Sept 11, 2020” by SounderBruce, CC BY-SA 4.0

Common questions

Delaware air quality FAQ

Which months have the worst air quality in Delaware?

Does wildfire smoke affect Delaware?

Why is air quality bad in Delaware in June?

What is the cleanest month for air quality in Delaware?

Where do I check the live AQI for Delaware today?

What does Unhealthy for Sensitive Groups actually mean?

EPA's definition, verbatim: "Members of sensitive groups may experience health effects. The general public is less likely to be affected." It covers AQI 101 to 150, shown in orange. Who counts as a sensitive group depends on the pollutant; for ozone EPA lists "People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults." The full pollutant-by-pollutant table is on this page, quoted from EPA's Technical Assistance Document.

Should I run or exercise outside today in Delaware?

Check the current category on AirNow, then apply EPA's cautionary statement for that category. For orange days and ozone, EPA's guidance reads: "Sensitive groups: Make outdoor activities shorter and less intense. Take more breaks. Watch for symptoms such as coughing or shortness of breath. Plan outdoor activities in the morning when ozone is lower." The equivalent statements for other pollutants are quoted in the scale section above.

How is this monthly calendar computed?

Satellite view of wildfire smoke filling the mountain valleys of central Idaho, with active fire detections marked in red — Wildfire smoke over central Idaho, NASA MODIS, September 2022. Smoke made a thousand miles away can override any local calendar for a week, which is why the live alert feed above matters even in an ozone state. “Smoke Shrouds the Sawtooth Range and Salmon River Mountains (MODIS 2022-09-12)” — MODIS Land Rapid Response Team, NASA GSFC — Public domain

Everywhere else

Air quality seasons in other states

Delaware tick forecastThe other thing to check before a long outdoor day, especially in brushy or grassy country.Delaware mosquito forecastWarm stagnant evenings that hold haze are the same evenings mosquitoes love.Air quality seasons nationwideSmoke, ozone, inversion and dust seasons across every state, on one page.

Monthly figures are computed from EPA Air Quality System daily AQI files (public domain regulatory data), 2020 to 2024, refreshed annually. Alerts are read live in your browser from api.weather.gov and reproduced as issued. AQI category definitions are EPA's, quoted verbatim. Today's measured reading is on EPA's AirNow page for Delaware.
Hero image: “Sunlight and Smog (Documerica)” — Gene Daniels, US EPA, NARA 552395 — Public domain