This is an article in which Thach summarizes everything he has spent his time trying to understand, research, and observe about air pollution. , not just Hanoi. There used to be a lot of concern about the air quality situation. After going to many different cities around the world, wherever he went, Thach carefully observed the air quality. The framework of this article will try to summarize in a general way by asking and answering questions.

1. What is the air pollution situation in Hanoi?

Answer: There are many types of air pollution, including toxic gas pollution, gas pollution harmful to health and polluted by dust. In Hanoi as well as neighboring provinces in the Northern Delta region, dust and fine dust pollution has emerged. Fine dust is dust particles that are very small in size, only a few micrometers. They can be anything, but because of their very small size, they float and fill the air. People often call fine dust PM2.5, which means Particulate Matter 2.5, which means fine dust with a diameter of less than 2.5 micrometers.

Fine dust concentration is converted to a scale value. If the good air quality level is <50, then the good level is from 50 to 100. The bad level is from 100 to 200, the level dangerous to health is from 200 to 300 and from 300 and above is a very dangerous level. for health. According to the daily observation and monitoring process of Hanoi at the time of writing the note (winter, early 2019), PM2.5 ranges from 150~220.

We still have very fresh, clean days, especially in the summer. And conversely, there are also days when pollution exceeds 300, let’s find out why in the next section below.

Air pollution in Hanoi

2. What is fine dust, how is it different from other types of dust? How does it affect health?

There are many types of dust. In many ways divided. Metal dust, aluminum dust, iron, copper, tin, mercury dust may also be present. Fabric dust from the textile industry is not small. Wood dust produced by industry. Plastic dust, sand dust, cement dust. There is also dust in liquid form. But generally persistent in the air are solid dusts.

Dust is divided by size. Large dust, coarse dust, sand dust, medium-sized dust, small-sized dust, pm20, pm10, pm5, pm2.5, pm1… Large, coarse and medium-sized dusts are heavy dusts and quickly settle. to the ground and can easily be swept away by the water. Road dust and construction dust are mostly this type of dust. Vehicles are often blown up or carried by strong winds. In general, this type of dust mainly causes dirt, discomfort, and immediate allergies, but does not affect health in the long term, except in cases where the concentration is too high, such as in the case of coal mine workers and cement production facilities. bamboo shoots, stones… As for most cases in urban areas, coarse to fine dust can easily be filtered by a mask and even without a mask, the respiratory system also has a prevention system. Automatically filter and eliminate dust.

Fine and ultra-fine dust. From 20um to less than 2.5um. This type of dust hangs a lot in the air, is small in size and light in weight, so it rarely settles. It floats and fills the air, easily creeping through door cracks, so the house is still affected. enjoy it even though it’s closed. This type of dust is the result of burning, wood burning, stubble burning, garbage burning, unfinished burning, engine exhaust gas, coal burning….

Ultrafine dust (the correct name for Pm2.5) is the type that affects health. Because their size is so small, they easily pass through the respiratory system’s filtration system, enter the capillaries and enter the circulatory system. Once they get past the dust blanket in the lungs, they enter the body and are very difficult to get out.

The danger of PM2.5 dust lies in this, it can enter the body but cannot exit. Anything that can go in but cannot come out is very harmful to the organism in the long term. The dust will be circulated by the blood and end up at the ends of the blood vessels. To tissues and become substances that hinder metabolism, become uninvited guests, which can cause difficulty in metabolism or problems in metabolism and mitosis. Fine dust travels through the blood to the brain and accumulates at the base of the skull over time, which can hinder information exchange, block access and cause Alzheimer’s dementia.

The danger of fine dust pollution is that it affects health in the long term, not cause acute problems. It’s like people who are addicted to cigarettes. Smokers also continuously inhale cigarette smoke into their lungs. They do not die immediately, nor do they get sick right away, but respiratory problems and other long-term problems will eventually come.

Fine dust in air pollution

3. Fine dust pollution How to observe?

Different from coarse dust and fine dust that can be observed such as road dust and industrial dust..Part Large amounts of fine and ultra-fine dust cannot be seen directly with the naked eye, but can be observed on a large scale because fine dust scatters light rays and makes the air opaque. With the appearance of high concentrations of fine dust, we will have a cloudy, cloudy atmosphere, making it impossible to clearly see buildings from several hundred to several kilometers away. Buildings that are more than 3km away sometimes cannot be seen.

A typical day of dust pollution in Hanoi

Fine dust can be observed using a trick based on the principle of light scattering. Use a strong, narrow-band light source in a dark room to clearly clarify the light scattering of fine dust as in the experiment below.

So on days when we see the air on the street is hazy, thanks to it not being completely foggy, we understand that that day is a day of fine dust pollution and consider the outdoor activities.

But it is also necessary to clearly distinguish the haze of fine dust pollution with the opacity of other common meteorological phenomena.


Dew often appears in autumn and winter, when the temperature at night is low but the sky is clear and sunny during the day. Differences in day and night temperatures and high humidity often cause morning dew.

Dew is essentially water vapor so it is not related to pollution. The characteristics of fog are that it is usually thick, pure white, and visibility in the fog is only a few dozen meters. And it usually only appears early in the morning, when the sun comes up it will disappear. And only appears when humidity is high.

So when it’s still foggy at noon, it’s not fog.

Low cloud phenomenon.

This phenomenon does not happen often but is not too rare in winter in areas near sea.

Specifically, the phenomenon is that the cloud concentration level lowers to near the ground on low-temperature days. But the humidity is very high due to the easterly monsoon. The weather is often overcast, leading to temperature inversions. Low clouds have no convection to rise to high altitudes, so they remain at low altitudes.

On days with low clouds, the cloud height can drop to 200~300m or even 100m or close to the ground. This means we will observe the clouds passing by over the city as if in Sapa or Da Lat.

Cause: The colder the air, the less water vapor it contains. Each level of air temperature can contain a certain amount of saturated water vapor, and the lower the temperature, the less this amount of water vapor can be contained. If the temperature is reaching saturated humidity, then if we lower the temperature, the excess water vapor will be forced to liquefy and condense into water particles. That is how clouds form and is also how mucus forms, which also explains why a glass of water taken out of the refrigerator sweats.

When the northeast wind blows strongly from the Gulf of Tonkin into the North, it brings with it a lot of steam. humid but the temperature is low, so the dew point temperature (dew point) increases. And when the actual temperature is equal to or lower than the dew point temperature, a very low cloud concentration phenomenon occurs.

The characteristics of low clouds are that they come in waves and clusters. When lying in the clouds, of course we see completely opaque white, visibility is very short. Sometimes it clears up, sometimes it becomes cloudy. On days with low clouds, the humidity is definitely >95% or more and the northeast wind blows east.

Low clouds at noon
Low clouds at night
Air pollution data
GEOS 5 remote sensing data on CO concentration distribution at a time. (from

Fine dust is generated through the process Burning organic substances, unfinished fires, burning fuels containing many impurities such as burning wood, burning straw, burning trash, burning straw, exhaust gases from gasoline engines, burning coal, etc. and fires such as house fires, forest fires, volcanic eruptions..

Extreme natural phenomena such as forest fires, volcanic eruptions as well as incidents such as house fires are rare and infrequent phenomena, so they are not the source of permanent pollution.

The sources of fine dust pollution in Hanoi can be classified into the following types: Endogenous, endogenous Locally born, neighboring endogenous, regionally endogenous and Externally invaded as follows:

  • Internal Local endogenous emissions include trash burning, straw burning, coal stove burning, and vehicle engine exhaust.
  • Neighboring endogenous sources include emissions of industrial parks, factories, and plants Industry has fossil fuel consumption activities such as coal – gasoline – oil, burning straw, burning bamboo to produce activated carbon.
  • The region’s endogenous resources include thermal power burning coal, oil, heavy industrial zones producing iron, steel, and cement.
  • Foreign invasion from neighboring areas, especially China, blows in with the northeast wind. China also has many megacities with populous populations and top-class industrial production, so it is obvious that China’s endogenous pollution is more obvious than ours, air pollution diffuses from this place to the wind. other places, so it is inevitable to be affected when the north receives weak northern winds.

A. Monitor through channels and websites reporting pollution index air. Current websites that provide fine dust pollution status include:

1/ Aqicn .org: Searching by Hanoi or Hochiminh city will show the air pollution index (AQI) at a few measuring stations in Hanoi and HCM. However, there are not many measuring stations and these measuring stations only reflect the local air surrounding the measuring station. To properly assess the pollution level of a large area, we need many measurement samples in different locations for comparison. That’s why we need to refer to other sources as well.

2/ This website updates many scattered measuring stations In the inner city of Hanoi, the downside is that the update is often one day late, which means it reports the pollution index of the previous day.

3/ PAM Air / software and web as well as a system of measuring stations throughout Hanoi (currently covering almost the entire country) and being developed Independently funded by private capital, using sensors that measure dust directly by the lightscattering method and connect to wifi to report fine dust index in real time.

4/ Air Visual: This channel uses data from a few Physical measuring stations combined with remote sensing data (not high accuracy) to build a model of fine dust pollution. There is reference value to visualize the change, but the numbers that Air visual gives sometimes do not reflect as accurately as the data from physical measuring stations from the sources listed above. I do not encourage people to trust this channel’s figures for the above reasons.

If we ever watch the media When news reports about air pollution in China, Mongolia, or India, we will see that they all have one thing in common: turbid air. This opaque layer is slightly gray in color, not white like dew or clouds. This layer of fog is not as thick as clouds, it is thinner, enough so that we can still see clearly close objects, but at farther distances it becomes blurry and at a distance it is completely opaque, like a thin mist. This layer of fog is called Smog – a combination of fog and smoke

The smog layer This is a layer of fine dust suspended along with other polluting gases with high specific gravity such as NO2, SO2, O3. Because they are dust mixed in the air, their specific gravity is heavier and therefore difficult to convect. They exist in a layer close to the ground and depending on the concentration and level of pollution that day, this layer can have variable thickness. Can range from a few hundred meters to kilometers or more. Based on what to judge the thickness of this fine dust layer?

Based on observations.

By simple geometry, the layer thickness can be determined This image in the photo is about 700m ~ 2 times the height of the Keangnam building.

The layer looks clear.
The gray pollution layer has an altitude that reaches the low cloud layer. Photo: Mr. Zoso Dung Nguyen
Layer looks clear.
A day of heavy pollution, PM2.5 index reached 260

Distinguishing characteristics when below ground level: distance vision is blurry, color is slightly dark, sky color is usually dark blue, when the air is polluted polluted, seen from the ground, the color of the sky is only blue or light blue. The more heavily polluted, the lighter the color becomes. And clouds on days when the air is clean are pure white and separate from the blue sky, but on polluted days, they are blurry, feeling like we are fading into a blue sky.

Another heavily polluted day, the dust layer is very thick, the sun sets very early even though there are very few clouds
Compared to a good AQI day, the air is cleaner. Even though it was a gloomy day

6. Why is the air polluted But there are still days when the sky is blue and the air is clean and fresh? What cleans the air?

Speaking about air cleaning agents, it must be affirmed that everything that belongs to the air is attached to it. The same goes for meteorology and air pollution. It is meteorological factors that affect pollution and in different directions. Listed as follows:


Widespread heavy showers can help wash away dust in the atmosphere and wash away harmful substances. Dust sticks to roads and trees, so the air will be fresher and the roads will also be cleaner. Heavy rain is also often accompanied by another most important factor that helps clean the air, which is WIND. Heavy rain also causes strong convection and contributes to the diffusion of pollution in the air layer close to the ground.

Slight rain, especially drizzle, will not be affected unless it is accompanied by wind.


WIND is the natural element that helps clean air pollution most effectively. WIND is like a broom, pushing polluted air elsewhere, diffusing it.

Strong wind days are clean air days. If the wind blows from a less polluted area, the air will be extremely clean. Air Quality Index can reach 15~30, which means it is as clean as on a remote island. If the wind is blowing from a polluted area, the air will naturally be dirty, but if it blows strongly, it is still cleaner than blowing lightly, because blowing strongly increases diffusion and convection.

For northern Vietnam: The largest source of endogenous pollution is the east: in the Quang Ninh region – Hung Yen – Hai Duong. And foreign pollution comes from the north, China. Because China is like Vietnam but on a more crowded urban scale, has stronger industrial activities and is also affected by similar climatic factors, in general they pollute and suffer more pollution than China. On polluted days when the wind blows from them to us, of course we receive more air pollution, although it is possible that along the way, the pollution is more or less diffused or retained by some areas. forest. But very little fine dust settles, so the forest does not retain much.

Thus, the law of pollution does not Winter’s air: Monsoon means the north-northeast wind blows: The first few days are strong and the sky is clear, gradually getting cloudier in the following days and the wind weakens and the air will be extremely dirty and last until the next season. next monsoon.

In the summer. The air will be extremely clean on windy days. That means west and southwest winds. Because the west and southwest are very clean, full of forests. To the west and southwest, the mountainous terrain is as high as a wall, and moisture and pollution are mostly trapped on the other side. Making the wind blow hot, dry but very clean, the AQI these days can be below 10, even in some places where it was measured at only 3~4 pm2.5 concentration units.

On windy days, the air will be polluted. On southeast wind or south wind days the air will be a little cleaner but not as clean as west wind days.

The sky is blue, the air is almost perfectly clean on windy summer days
Hanoi on a day with absolutely fresh air just experienced a thunderstorm with very strong winds, all types of pollution were blown away. After flying, the sky was so clear that the Ba Vi mountain range 40km away was still clear.


Clearly, convection plays a very important role in the diffusion of pollution. Convection creates air currents that move vertically, stir up wind and diffuse pollution. That’s why in the summer, on hot sunny days, the air is often cleaner because high temperatures promote convection and winds.

Heat inversion?

Contrary to convection, heat inversion is a meteorological phenomenon between non-convective air. This phenomenon often occurs in winter, on cloudy days, the ground temperature is low while the sun heats the air above, causing the air to be cold and hot, and therefore cannot occur. out convection. Pollution is also trapped, even compressed close to the ground.

That’s why on winter days it’s dark, windy, and the air is very dirty.

7. Is only Hanoi suffering from air pollution? Will leaving Hanoi mean we have fresh air?

The answer is not necessarily.

Let’s look back at section 4. talk about the sources of fine dust pollution: The sources of fine dust pollution in Hanoi include the following types: Endogenous, locally endogenous, neighboring endogenous, regional endogenous and foreign invasion.

Among these factors, regional endogenous is considered Mentioned as a factor causing fine dust pollution for the entire territory, it is not limited to an area such as a city. And the agents that cause such widespread pollution are large-scale fuel burning activities, which here are coal-fired thermal power plants and heavy industrial activities using coal or oil.

Among fuels, coal is the type with the lowest cost to produce electricity, so thermal power Coal is widely used in the current period of hot industrial development. But coal is the most polluting fuel, especially creating ash and fine dust. The amount of dust created when burning coal is more than burning oil, more than gasoline, and more gasoline than burning gas. Burning gas theoretically does not produce fine dust because gas is a fuel without a substrate or solid impurities to produce smoke or fly ash.

Exhaust gas towers are raised high at thermal power plants and combined with the hot gas rising rising high causes a huge amount of dust to be lifted up and then slowly diffuse and sink down. That is why fine dust pollution from a factory sometimes does not have the heaviest impact on the factory area itself, but rather spreads across the factory area. wide area and sometimes the impact is more severe at a distance from the plant in the direction of the wind.

Remote sensing data and other data show that fine dust pollution does not occur locally but takes place on a regional scale. Pollution is not only found in Hanoi, that is a misconception. Sometimes when we go to Hung Yen, Nam Dinh or back to Vinh Phuc, we still get hazy atmosphere and almost the same AQI index.

Only the northwest region has high terrain, the spread of air pollution at low levels sometimes Cannot cross mountain ranges so it is difficult to affect high mountainous areas. Therefore, the air in the Northwest is clean almost all year round.

Based on my own experience through a long process of observation including observation through gas software Visualization, remote sensing and fieldwork, in Vietnam, the year-round clean areas are the central region, south central region, followed by the northwest and central highlands. Next is the North Central region, the Mekong Delta and then the Southeast and the least clean is the North. The capital Hanoi and Ho Chi Minh City will be more polluted partly because of vehicle exhaust and human activities related to burning.

Every year, during the crop season, the whole country burns rice straw, sometimes causing serious local pollution. important in the Northern Delta region.

Real-time CO concentration level at a time. CO is a gas produced by incomplete oxidation of organic substances. It reflects human fuel burning activities and we see that the distribution of air pollution is on a regional scale, not on a regional scale. not local.

8. What can we do to reduce fine dust pollution? Will planting more trees help reduce dust pollution?

To reduce fine dust pollution, we have 2 groups of work:

1/ Reduce fine dust emissions, reduce air pollution

As analyzed, fine dust is mostly released into the environment through burning, burning of fossil fuels or other substances. Organic contains many impurities. Therefore, reducing fine dust emissions means reducing the burning of fuels such as coal, firewood, straw, and garbage.

If straw burning takes place only twice a year, then coal burning is continuous every day and the amount of coal that must be burned to serve energy production also accounts for a proportion that overwhelms other sources.

Therefore, solutions to reduce fine dust emissions need to aim at limiting gradually coal-fired power. Gradually replace coal-fired power with other cleaner power sources such as gas power and renewable energy, even nuclear power. This is a long story and a mind-numbing topic, but at least in terms of social awareness, we need to recognize that for the sake of the environment, coal-fired power in the future needs to be controlled and gradually replaced.

Supercities with a population of several tens of millions of people are also sources of dust emissions smooth and significantly polluted when there are millions of vehicles running on gasoline/oil. In the future, it is necessary to refer to Western environmental laws and follow them in limiting high-emission vehicles, limiting oil-powered vehicles, and gradually prioritizing and encouraging gas-powered vehicles. / electric/hybrid gasoline electric..

Wind is a factor that helps diffuse pollution, so Thach There is a separate advisory for the electricity industry to avoid building new or gradually retiring coal-fired power plants in areas with little wind such as the North, and replan them to localities with strong winds all year round. Thermal power plants surrounding the northern delta region, which has static winds and often easterly winds, causes pollution to accumulate and affect the entire northern delta region, which has a very large population.

2/ Filter out dust.

China and other countries advancing in the fight against fine dust pollution have installed filtration towers air in public areas. China has operated the world’s largest air filtration towers in Shaanxi that can ensure air improvement for an area of 10 square kilometers. These are very practical actions of the government to protect the environment and people’s health.

The world’s largest air filtration tower in Shaanxi, China

In addition, according to Thach’s idea, the Hanoi government can install filters or Small air filtration towers and large filtration towers in parks and densely populated areas to improve local air quality in each area.

Planting many urban trees is not a good solution to limit fine dust.

Green plants hold dust and do not blow dust away, so plants do not help clean fine dust. At the same time, trees in urban areas eliminate wind – an agent that helps diffuse pollution. So pollution tends to accumulate and stay. There have been many studies showing the opposite effect of urban trees on limiting dust pollution. Even in other issues such as creating shade, balancing temperature, creating oxygen, trees have positive effects.

9. What can each individual do to limit the effects of fine dust pollution? helps reduce air pollution

For each of us, to reduce the impact of fine dust , we have 2 jobs:

1/ Monitor updates on fine dust pollution every day Apps: Air visual / Pam Air… With red warning days about fine dust concentrations, if we have to go out on the street other than sitting in a closed car, we need to wear good masks, Sealed, can filter fine dust. Talking about masks, medical masks are completely useless because they have many gaps. Regular masks also cannot limit fine dust when there are many gaps. Without an air valve, each time you inhale and exhale dust will flow with the air through the gap between the mask and your face. You should use a mask that when worn is sealed against the face, has a metal nose clip, has a one-way breathing valve and is advertised as being able to filter out fine dust.

2/ Close the door during red warning days about fine dust pollution. There should be an air purifier at least in the bedroom. Talking about air purifiers. The purifier is essentially a fan that sucks air through filters. A good air purifier is a machine with 3 membranes, coarse filter – HEPA membrane to filter dust and fine dust – activated carbon membrane to filter out organic impurities. A good air purifier must have a sensor that fully measures temperature – humidity – and fine dust concentration in the air and updates it on the screen or via the phone app. A standard air purifier is one that can have its filters replaced regularly. Usually every 6 months because activated carbon has its lifespan, and the filter membrane will become worse after filtering for a long time.

The filter reports the fine dust pollution value on the phone app

10. Types of air purifiers, which type should I use?

Currently there are basically 2 types of air filtration: Active filtration with dust filter and passive air ionization filter.

Active filtering:

Are air purifiers that blow or vacuum dust through the dust filter membrane and thus the dust is retained in the filter membrane, similar to water filtration. The basic structure of an active air purifier is a blowing or suction fan, a filter and the machine’s frame. This filter membrane structure usually consists of 3 layers: Coarse mesh to block foreign objects – HEPA / EPA layer to filter >99% of dust in the air – Activated carbon layer to adsorb organic substances and odors suspended in the air. air. So the center of this filter is the HEPA / EPA layer. HEPA stands for High Efficient Particulate Air and is a super effective air filter. Filters 99.95% of PM2.5 dust. Some purifiers on the market are only equipped with EPA (Efficient Particulate Air) filters that can filter about 98 ~ 99% of PM2.5 dust, which is slightly less effective than HEPA. Current purifiers may have additional features such as moisture compensation or air ionization.

Are air purifiers that do not ionize gas, in other words electrifying components suspended in the air. When dust particles suspended in the air are electrified, they tend to attract surrounding particles with electrostatic attraction, and thanks to that, the dust particles become larger, heavier and gradually settle to the ground. Electrostatic suspended particles also have the effect of killing bacteria suspended in the air.

So negative ion purifiers are different from active air purifiers in that negative ion purifiers causes dust to settle on the floor, while active air purifiers keep dust in the filter.

The effectiveness of a negative ion purifier is not as effective as an active filter, but it contributes to killing bacteria because So some air purifiers actively integrate air ionization into one.

* * When buying an air purifier, be careful not to buy a used one or use an old filter that is no longer effective. Make sure the filter is brand new. After a while, the filter also accumulates dust because it loses its ability to filter air, so the filter must be replaced periodically, usually from 6 months to 1 year depending on the manufacturer.