Kathmandu AQI Explained: How to Read the Number

If you have looked up Kathmandu's air quality, you have probably seen a single number — 80, 150, sometimes over 300 — color-coded green, orange, or red. That figure is the Air Quality Index, or AQI, and learning to read it is one of the most useful travel skills for a visit to the Nepali capital. This guide explains exactly what the Kathmandu AQI number means, how the scale works, and which readings should actually change your plans.

For the broader story of why Kathmandu's air gets dirty and how it affects health, see our companion guide on Kathmandu air quality. Here, the focus is narrower and more practical: how to read the number itself.

Key takeaways

• AQI is a single number on a 0-to-500 scale that converts pollution data into an easy-to-read health rating — higher means dirtier air.
• The scale has six color-coded bands, from "good" (0–50, green) to "hazardous" (301+, maroon).
• For Kathmandu, the driving pollutant is almost always PM2.5, fine particles that lodge deep in the lungs.
• In 2025 Kathmandu's annual average mapped to an AQI of about 109, but daily readings swing wildly with season and time of day.
• Most apps used in Nepal default to the US EPA scale, which is the one explained here; other countries' scales differ.
• A high reading means adjust — mask up, move activity indoors or to the hills — far more often than it means cancel.

What the AQI number actually is

The Air Quality Index is a translation tool. Air pollution is measured in technical units — micrograms of particles per cubic metre of air — that mean little to most people. The AQI takes those measurements and converts them into a single number on a fixed scale, so that "is the air okay today?" has a quick answer.

In the system used by most trackers in Nepal, that scale runs from 0 to 500, and the rule is simple: the higher the number, the more polluted the air and the greater the health concern, according to the US EPA's AirNow program, which maintains the scale. A reading of 40 is clean; a reading of 250 is genuinely dangerous to be out in for long.

Crucially, the AQI is reported for whichever pollutant is worst at that moment. In Kathmandu, that is nearly always PM2.5 — fine particulate matter under 2.5 microns wide, small enough to penetrate deep into the lungs and enter the bloodstream. When you read a Kathmandu AQI figure, you are almost always reading a PM2.5 number in disguise.

The six AQI bands, decoded

The whole scale is divided into six categories, each with a color and a plain-language health message. This is the heart of reading the number: once you know which band today's figure falls into, you know what to do.

| AQI range | Category | Color | What it means for you | |---|---|---|---| | 0–50 | Good | Green | Air is clean; do anything outdoors. | | 51–100 | Moderate | Yellow | Fine for most; unusually sensitive people may notice. | | 101–150 | Unhealthy for sensitive groups | Orange | Children, older adults, and those with conditions should ease off. | | 151–200 | Unhealthy | Red | Everyone may feel effects; limit prolonged outdoor exertion. | | 201–300 | Very unhealthy | Purple | Health alert; avoid outdoor activity, mask if you must go out. | | 301–500 | Hazardous | Maroon | Emergency conditions; stay indoors with windows closed. |

These breakpoints come from the US EPA, which tightened the scale in 2024 so that a PM2.5 level of 9.0 micrograms per cubic metre now marks the top of the "good" band (it was 12.0 before). The categories are based on scientific evidence linking particle concentrations to health risk, so they are not arbitrary — each step up genuinely reflects a higher level of harm.

A quick mental shortcut

If you remember nothing else, remember the round numbers:

• Under 100 — generally a good-to-okay day. Plan freely.
• 100 to 150 — sensitive travelers should be cautious; everyone else is fine.
• 150 to 200 — a bad-air day; everyone should ease back and consider a mask.
• Over 200 — a serious day; minimize time outside and protect yourself actively.
• Over 300 — hazardous; treat it as a stay-indoors situation.

What counts as a "normal" Kathmandu reading

Numbers are easier to interpret with local context. Kathmandu does not have one typical AQI — it has a range that shifts dramatically across the year, which is why a live check beats any rule of thumb.

For the year as a whole, IQAir's 2025 data put Kathmandu's average PM2.5 at about 39 micrograms per cubic metre, which maps to an AQI of roughly 109 — in the "unhealthy for sensitive groups" band. That is the annual average, though, and real days look very different from it:

• Monsoon days (Jun–Sep) can drop into the green or yellow, with rain scrubbing the air clean.
• Winter mornings (Dec–Feb) often sit in the orange-to-red range as cold, still air traps smog in the valley bowl.
• Spring fire season (Mar–May) produces the extremes: during the wildfire surge of 1 April 2025, valley PM2.5 peaked around 365 micrograms per cubic metre — deep into the hazardous band — and Kathmandu briefly ranked the most polluted city on earth, according to the Kathmandu Post.

So a reading of 90 is a genuinely good Kathmandu day; a reading of 180 is a typical bad winter morning; and anything above 300 is the kind of episode that makes the news. Our month-by-month weather guide and best time to visit Nepal can help you anticipate which end of the range your dates fall into.

Why two apps can show different numbers

A common confusion: you check two trackers and get two different AQI values for the same morning. There are two honest reasons for this.

First, different countries use different AQI scales. The US EPA scale described here is the most common default, but India, China, and others use their own breakpoints, so identical air can produce a different index number depending on which standard an app applies.

Second, readings are local. Pollution varies across the valley, so a sensor in one neighborhood can legitimately differ from one a few kilometers away. For consistency, pick one reputable source — IQAir is the most widely cited for Kathmandu — and watch the trend and band rather than fixating on a single exact digit.

Turning the number into a plan

Reading the AQI only helps if it changes what you do. Here is how each band translates into action on a Kathmandu trip.

When the number is under 100

Green light. This is the time for walking tours, outdoor sightseeing, and day trips. Make the most of clean-air days for anything strenuous — early in your trip, or whenever the monsoon delivers a clear morning.

When the number is 100 to 200

Adjust rather than retreat. If you are in a sensitive group — a child, an older traveler, pregnant, or managing asthma or heart or lung conditions — take it easy, carry any medication, and favor indoor plans. Everyone else can carry on, ideally with a well-fitted N95 or KN95 mask for long stretches outdoors in traffic. Cloth and surgical masks do little against PM2.5.

When the number is over 200

Take it seriously. Minimize time outdoors, keep an N95 on if you must be out, and shift to indoor attractions — museums, cafes, indoor markets. This is also the moment to consider leaving the valley entirely. The hills around Kathmandu trap far less pollution, so a night at a ridge town like Nagarkot or Dhulikhel, or a day trip to Bhaktapur, can swap a hazardous reading for fresh air. Structuring your days with flexible Kathmandu day tours makes it easy to pivot indoors or out of town when the number climbs.

The bottom line on reading Kathmandu's AQI

The Kathmandu AQI is not a mystery once you know the system: a 0-to-500 number, six color bands, almost always driven by PM2.5, with under 100 meaning a good day and over 200 meaning a serious one. Pick one trusted tracker, glance at it like the weather, and let the band — not your assumptions — guide whether you walk the old city, mask up, or head for the hills. With that one habit, Kathmandu's variable air becomes something you manage easily rather than something that catches you out. For the full health and seasonal picture behind these numbers, read on in our Kathmandu air quality guide.

Sources

• Air Quality Index (AQI) Basics — AirNow (US EPA)
• IQAir implements 2024 update to U.S. EPA Air Quality Index (AQI) — IQAir Knowledge Base
• Kathmandu Air Quality Index (AQI) and Nepal Air Pollution — IQAir
• Raging wildfires drive pollution surge in Kathmandu — The Kathmandu Post (Apr 2025)
• The National Ambient Air Quality Standards for Particle Pollution — US EPA