Air Quality Index and PM2.5: How to Read the Numbers and Protect Your Health

Open any weather app in Vietnam and you will find one more number worth checking besides temperature and rain: the air quality index. Some mornings the sky looks foggy when it is actually haze; other days the sun shines while the index sits in the unhealthy range. Learning to read AQI and understand fine particulate matter, or PM2.5, lets you protect your health with facts rather than guesswork.

What exactly is AQI?

The Air Quality Index converts concentrations of major pollutants — fine particles (PM2.5), coarser dust (PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide — into a single scale from 0 to 500. The higher the number, the dirtier the air. At any given moment, the index reflects whichever pollutant is worst, and in Vietnamese cities that is almost always PM2.5.

Keep in mind that AQI is a translation of monitoring data, not a direct measurement, and different apps may use the US, European or Vietnamese conversion scales. Do not worry if two apps disagree by a few dozen points; what matters is the color band and whether the trend is rising or falling.

The color scale at a glance

• Green (0–50) — Good: clean air, enjoy the outdoors freely.
• Yellow (51–100) — Moderate: fine for most people; unusually sensitive individuals should pay attention to how they feel.
• Orange (101–150) — Unhealthy for sensitive groups: children, the elderly and people with heart or lung conditions should ease off strenuous outdoor activity.
• Red (151–200) — Unhealthy: everyone may start to feel effects; sensitive groups feel them strongly.
• Purple (201–300) — Very unhealthy: health alert, limit time outside.
• Maroon (above 300) — Hazardous: emergency conditions, stay indoors with windows closed.

A good habit: start adjusting your plans at orange, not red.

Why PM2.5 deserves special attention

PM2.5 refers to particles measuring 2.5 micrometers or less — roughly thirty times thinner than a human hair. That tiny size is exactly what makes them dangerous. Your nose and throat cannot filter them out, so they travel deep into the alveoli, where your lungs exchange oxygen, and some pass into the bloodstream, reaching the heart, brain and other organs.

Short-term exposure at high levels causes coughing, itchy throat, stinging eyes and shortness of breath. Long-term exposure raises the risk of asthma, chronic lung disease and cardiovascular problems, and can affect lung development in children. In Vietnam, the main sources are traffic exhaust, construction dust, industry, post-harvest straw burning, open waste burning and coal stoves.

Why winter air is often worse

Traffic runs year round, so why does the index spike every winter across northern Vietnam? The answer lies in the weather:

• Temperature inversion: normally, warm air near the ground rises and carries pollution away. On clear, calm winter nights, the ground cools quickly and the air near the surface becomes colder than the air above it, forming a lid that traps smoke and dust at street level.
• Weak wind and little rain: dry winter weather means pollution is neither washed out nor blown away.
• More sources: year-end construction, traffic and straw burning all add up.

That is why from roughly October to March, Hanoi and the surrounding Red River Delta often see pollution episodes lasting several days, usually worst in the early morning and easing once the sun and wind pick up. In the south, Ho Chi Minh City has its own hazy spells during the dry season when the air stays still.

Who should be extra careful

Everyone breathes the same air, but some people feel the effects sooner and harder:

• Young children, who breathe faster and whose lungs are still developing
• Pregnant women
• Older adults and anyone with asthma, chronic bronchitis or heart disease
• People who work outdoors for long hours — ride-hailing drivers, construction workers, street vendors

If anyone in your family belongs to these groups, checking the AQI each morning should be as routine as checking for rain.

Masks and purifiers that actually work

Cloth masks and standard surgical masks block large dust and droplets, but they do little against PM2.5 — the particles are too small and the masks do not seal against your face. For real protection, choose masks rated N95, KN95 or FFP2, fitted snugly over the nose and chin, and replace them when damp or dirty. Children need properly sized versions.

For indoor air purifiers, three things matter most: a true HEPA filter, enough capacity for your room size, and regular filter replacement. Put the unit in the bedroom, where you spend the most hours, and keep the door closed while it runs. On bad-air days, avoid opening windows during rush hour and wipe floors with a damp cloth instead of dry sweeping. Houseplants look lovely, but their contribution to removing fine particles is negligible.

When to skip the outdoor workout

Exercise makes you breathe far more air than at rest, which means far more particles reaching your lungs. As a rule of thumb: at orange, sensitive groups should move workouts indoors while everyone else lowers intensity and stays away from busy roads; at red, shorten outdoor time and postpone that run; at purple or above, stay inside with the purifier on and wear a properly rated mask if you must go out.

Air quality is typically worst in the calm early morning and at night, improving around midday and after rain — so shifting your workout time is often smarter than skipping it altogether.

Small habits, big difference

Air pollution is no reason to hide indoors all winter, but it does reward a little daily awareness. A quick glance at the index, a proper mask in your bag and a flexible exercise schedule go a long way. Before heading out each morning, take a minute to check the forecast for your city so neither the weather nor the air catches you by surprise.