Boston's Air Stayed Clean This Summer — But Not Always

For most of late July and August 2026, Boston's air was genuinely good. The numbers confirm it. Across 29 days of continuous monitoring, the city's AQI averaged just 28.8 — well within the "Good" band on the US AQI scale. But averages can obscure what actually happened on the ground. A closer look at the daily record reveals a city that swung between its cleanest readings of the season and a brief but notable pollution peak, with a curious PM10 spike that stands apart from everything else in the dataset. This is what the data says, day by day.

A Quick Look at the Numbers

The table below captures the key metrics across the full monitoring window, from July 30 to August 27, 2026.

Metric	Start Value	End Value	Average	Minimum	Maximum	Net Change
AQI	24	26	28.8	16 (Aug 4)	42 (Aug 7)	+2
PM2.5 (µg/m³)	5	6	6.6	3 (Aug 4)	10 (Aug 7)	+1
PM10 (µg/m³)	14	13	14.8	12 (Aug 18)	30 (Aug 20)	-1

The net changes across all three metrics are nearly flat — just +2 for AQI, +1 for PM2.5, and -1 for PM10. On paper, Boston ended August almost exactly where it began. But the journey between those two endpoints is where the real story lives.

Daily AQI trend generated from the air-quality history.

Daily particulate matter trend for PM2.5 and PM10.

The Early August Surge: A Sharp Rise, Then a Sudden Drop

The most striking feature of this dataset is the sequence that unfolded between August 1 and August 7. It is the most volatile stretch in the entire 29-day record.

Boston opened the monitoring window quietly. On July 30, the AQI sat at 24 with PM2.5 at just 5 µg/m³ — textbook clean air. Then, almost without warning, August 1 brought an AQI of 36, driven by PM2.5 climbing to 8 µg/m³. The city held in that elevated range for two more days: AQI 34 on August 2, and 33 on August 3. These readings are still technically "Good" by US standards, but they represent a meaningful step up from the baseline.

Then came August 4 — the cleanest single day in the entire dataset. AQI collapsed to 16. PM2.5 fell to just 3 µg/m³. This kind of sharp one-day dip typically signals a strong weather system moving through, flushing stagnant air with wind or rain. It was a brief reprieve.

What followed was the dataset's defining moment. August 6 pushed AQI to 38 and PM2.5 to 9 µg/m³. Then August 7 hit the peak: AQI 42, PM2.5 at 10 µg/m³. This was the highest reading of the entire monitoring period. While 42 still falls within the "Good" category, it sat at the upper edge — close enough to the "Moderate" threshold of 51 to warrant attention, particularly for sensitive groups such as children, the elderly, and those with respiratory conditions.

The rebound was just as fast. By August 8, AQI had fallen back to 29. By August 10, it was at 28. The early August surge lasted roughly a week in total, with its sharpest pressure concentrated in just two days.

Mid-August: Stability With Brief Interruptions

After the early August volatility settled, the city entered a more stable phase. From August 8 through August 17, Boston's AQI oscillated within a relatively narrow band, mostly between 22 and 35. There were no dramatic single-day swings comparable to what happened around August 4 and 7.

A few dates stand out within this quieter stretch. August 11 saw AQI tick back up to 33 with PM2.5 at 8 µg/m³, suggesting a minor recurrence of the conditions that drove the earlier peak — but without the same intensity. August 15 brought another clean-air day, with AQI at 22 and PM2.5 dropping to 5 µg/m³. August 16 then reversed that gain immediately, jumping to 35 with PM2.5 back at 8 µg/m³.

This back-and-forth pattern — a cleaner day followed quickly by a moderate rise — is characteristic of summer air quality in northeastern US cities. Afternoon heat builds surface ozone and fine particle concentrations, while overnight cooling and morning winds can temporarily clear the air. The data reflects that rhythm clearly across this ten-day window.

August 17 brought another dip to 22, and by August 18 the AQI settled at 25 with PM2.5 at 6 µg/m³. PM10 on August 18 reached its lowest point in the entire dataset at just 12 µg/m³, suggesting particularly clean coarse particle conditions on that day.

The August 20 PM10 Anomaly: A Number That Demands Explanation

The single most unusual data point in this entire 29-day record has nothing to do with PM2.5 or AQI. It is the PM10 reading on August 20: 30 µg/m³.

To understand why this matters, consider the context. PM10 averaged just 14.8 µg/m³ across the full monitoring period. On most days, it hovered between 12 and 15 µg/m³ — an extremely narrow and consistent range. Then on August 20, it doubled the typical value in a single day, reaching 30 µg/m³. The day before, PM10 was 14. The day after, it was back to 14.

What makes this anomaly particularly interesting is what the AQI and PM2.5 did on the same day. AQI on August 20 was 35 — elevated, but not dramatically so. PM2.5 was 7 µg/m³, only slightly above average. This divergence is telling. When AQI and PM2.5 remain moderate while PM10 spikes sharply, it typically points to a coarse particle event rather than a fine particle or combustion-driven pollution episode. Possible causes include construction dust, road resuspension, a localized soil disturbance, or even a brief wind event carrying coarser particulate matter into the monitoring area.

For Boston residents, this distinction matters. PM10 particles are generally less dangerous than the finer PM2.5 particles, which penetrate deeper into the lungs. But a doubling of coarse particle levels in a single day is still worth noting, especially for those with asthma or other respiratory sensitivities who spend time outdoors. The spike was isolated and short-lived, but it is the sharpest single-metric movement in the entire dataset.

The final week of the monitoring period — August 21 through August 27 — was largely calm. AQI values ranged from 17 to 35, with August 26 recording the second-lowest AQI of the entire period at just 17. PM2.5 stayed between 4 and 8 µg/m³. PM10 returned to its baseline range of 13 to 14 µg/m³. The city closed the monitoring window on August 27 with AQI at 26 and PM2.5 at 6 — almost identical to where it began on July 30.

What This Data Means for Residents and the Broader Picture

For anyone tracking Boston's air quality in the context of India's own pollution challenges, the contrast is striking. An average AQI of 28.8 over nearly a month would represent an exceptional stretch even in India's cleanest hill stations, let alone its major urban centres where AQI readings in the hundreds are common during winter months. Boston's worst day — AQI 42 on August 7 — would qualify as a good air quality day in cities like Delhi, Mumbai, or Kolkata during peak pollution season.

That said, this data carries useful lessons regardless of geography. The August 4-to-7 sequence — a sharp clean day followed immediately by the period's worst readings — illustrates how quickly air quality can shift even in cities with strong environmental controls. Weather systems, wind patterns, and regional transport of pollutants can override local emission reductions in a matter of hours. Boston's monitoring infrastructure captured that shift in real time, which is precisely what granular daily data is designed to do.

The PM10 spike on August 20 reinforces another important principle: AQI alone does not always tell the complete story. A day with a moderate AQI can still carry unusual coarse particle loads that affect specific populations. Residents with respiratory conditions should track individual pollutant readings, not just the composite index number.

Across the full 29-day window, Boston demonstrated what a city with well-managed air quality looks like in summer — predominantly clean, occasionally elevated, and capable of recovering quickly from short-term spikes. The AQI trend across this period ended almost exactly where it started, with a net change of just +2. But the path between those two data points contained enough variation to tell a meaningful story about how urban air quality actually behaves when you watch it closely, day after day.