Air quality
Air quality in Waltham today
Updated
Moderate (US AQI 66) Acceptable for most people. Unusually sensitive people should consider shortening long or heavy exertion outdoors. Model estimate for Sun, Oct 25, 10:00 pm (02:00 UTC).
Live AQI, PM2.5, PM10 and ozone for Waltham, Massachusetts, United States, with a 48-hour forecast and the best time to be outside.
- US AQI now
- 66 Moderate
- PM2.5
- 12 µg/m³ 0.8× WHO 24-h guideline
- PM10
- 12 µg/m³
- Ozone
- 18 µg/m³
- Dominant pollutant
- PM2.5 sub-index 66
- Best hour tomorrow
- 7:00 am AQI 54
- European AQI
- 59
Air quality in Waltham right now
Right now the modelled US Air Quality Index for Waltham, Massachusetts, United States is 66, which falls in the "Moderate" band (up to 100). Acceptable for most people. Unusually sensitive people should consider shortening long or heavy exertion outdoors.
The index is driven mainly by PM2.5, whose own sub-index is 66. Fine and coarse particles come from traffic, heating, industry, dust and smoke; they build up on calm nights and clear when wind or rain arrives.
PM2.5 is 12 µg/m³, within the WHO 24-hour guideline of 15 µg/m³; PM10 is 12 µg/m³ against a guideline of 45; ozone is 18 µg/m³ (below the WHO 8-hour level of 100); nitrogen dioxide is 58 µg/m³ (above the 24-hour guideline of 25).
Also this hour: carbon monoxide 288 µg/m³.
NASA FIRMS shows 5 fire hotspots within 300 km in the last day (nearest about 200 km), but particle levels are not elevated enough to point to smoke right now.
Waltham AQI forecast: next 48 hours
Over the next 24 hours the AQI is forecast to range from 54 around Mon 7:00 am to 66 around Sun 10:00 pm, staying "Moderate" throughout.
The following day is expected to be cleaner, averaging about 48 against 57 for the next 24 hours.
| Local time (America/New_York) | US AQI | Category | PM2.5 µg/m³ | PM10 µg/m³ | Ozone µg/m³ |
|---|---|---|---|---|---|
| Sun 10:00 pm | 66 | Moderate | 12.1 | 12 | 18 |
| Mon 1:00 am | 61 | Moderate | 12.3 | 13 | 34 |
| Mon 4:00 am | 56 | Moderate | 13.2 | 13 | 46 |
| Mon 7:00 am | 54 | Moderate | 16.9 | 17 | 31 |
| Mon 10:00 am | 55 | Moderate | 15.3 | 16 | 49 |
| Mon 1:00 pm | 56 | Moderate | 8.4 | 9 | 89 |
| Mon 4:00 pm | 56 | Moderate | 7.7 | 8 | 89 |
| Mon 7:00 pm | 57 | Moderate | 14.4 | 15 | 49 |
| Mon 10:00 pm | 57 | Moderate | 15 | 15 | 29 |
| Tue 1:00 am | 57 | Moderate | 8.4 | 8 | 48 |
| Tue 4:00 am | 56 | Moderate | 3.2 | 3 | 50 |
| Tue 7:00 am | 53 | Moderate | 3.9 | 4 | 36 |
| Tue 10:00 am | 50 | Good | 2.4 | 3 | 53 |
| Tue 1:00 pm | 43 | Good | 2.4 | 3 | 70 |
| Tue 4:00 pm | 39 | Good | 3.3 | 4 | 72 |
| Tue 7:00 pm | 37 | Good | 24.2 | 25 | 27 |
| Tue 10:00 pm | 47 | Good | 20.5 | 21 | 8 |
Every third hour of the CAMS forecast via Open-Meteo. US AQI uses rolling averages (24 h for particles, 8 h for ozone), so it moves more slowly than the hourly concentrations.
Best time to be outside in Waltham tomorrow
Between 6 am and 9 pm local time, the cleanest air tomorrow is forecast around 7:00 am (AQI 54, Moderate) and the worst around 5:00 pm (AQI 57). The difference across the day is small, so timing matters little.
Waltham vs the rest of United States
Across 747 United States cities we track, the average US AQI right now is 52 (Moderate). Waltham at 66 is 14 points worse than average.
It ranks 660 of 747 from cleanest to most polluted; the cleanest is Hilo (22) and the most polluted Springfield, PA (132).
For context, the median across 3,909 cities worldwide is 57.
Air quality in cities near Waltham
| City | Distance | US AQI | Category | PM2.5 µg/m³ |
|---|---|---|---|---|
| Watertown | 4 km | 69 | Moderate | 13.9 |
| Belmont, MA | 5 km | 69 | Moderate | 13.9 |
| Brighton, MA | 7 km | 69 | Moderate | 13.9 |
| Arlington, MA | 8 km | 69 | Moderate | 13.9 |
| Lexington, MA | 8 km | 69 | Moderate | 13.9 |
| Cambridge, MA | 11 km | 69 | Moderate | 13.9 |
| Medford, MA | 12 km | 69 | Moderate | 13.9 |
| Winchester, MA | 12 km | 69 | Moderate | 13.9 |
Model vs measured
The nearest measured PM2.5 reading is 2.8 µg/m³ from Sensor.Community (median of 3 sensors), centred about 23 km from the city centre. That is 9.3 µg/m³ below the model estimate of 12 µg/m³. Low-cost sensors can read high in humid air; reference monitors are more accurate but sparser.
Advice for sensitive groups
At 66, most people are fine outdoors. If you are unusually sensitive to PM2.5, keep long or intense workouts to the cleaner hours shown above.
What the US AQI numbers mean
| US AQI | Category | What it means |
|---|---|---|
| 0-50 | Good | Air quality is satisfactory and poses little or no risk; a good time for outdoor activity. |
| 51-100 | Moderate | Acceptable for most people. Unusually sensitive people should consider shortening long or heavy exertion outdoors. |
| 101-150 | Unhealthy for Sensitive Groups | People with heart or lung disease, older adults, children and teens should cut back on prolonged or heavy exertion outdoors. |
| 151-200 | Unhealthy | Everyone should reduce prolonged or heavy exertion outdoors; sensitive groups should avoid it and keep windows closed. |
| 201-300 | Very Unhealthy | Health alert: everyone should avoid prolonged or heavy exertion outdoors, and sensitive groups should stay indoors. |
| 301-500 | Hazardous | Emergency conditions: everyone should avoid all outdoor physical activity and stay indoors with filtered air if possible. The scale tops out at 500; heavier dust or smoke is shown as 500. |
Values on this page are modelled by CAMS (Copernicus) and served by Open-Meteo; they are estimates for the area around the city centre, not readings from an official monitor.
Nearby cities
More for Waltham
Air quality elsewhere
Data: Open-Meteo Air Quality API, Copernicus Atmosphere Monitoring Service (CAMS), Sensor.Community, NASA FIRMS.