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What is
air pollution?

Lisa, aged 16

Air pollution occurs when harmful substances derived from all kinds of sources end up in the air. Inhaling polluted air can damage our health. Lisa (16) likes jogging and playing football, but wonders if it is actually healthy to exercise in the open air. Although the air quality has improved considerably in recent years, we can still do a lot ourselves to eliminate air pollution.


What is air pollution?

We talk about air pollution when harmful substances derived from all kinds of sources end up in the air. These substances are released into the air via factories, but also through the exhaust of cars, or from the chimney on the roofs of houses. Inhaling polluted air can harm your health. The target of the European Union (EU) and the World Health Organisation (WHO) is to achieve the best air quality possible which provides the highest protection to the population. To achieve this goal they set certain guidelines.

If you want to find out more on these guidelines click here.

Where does air pollution come from?

Traffic, industry, agriculture and other human activities are the main causes and sources of air pollution in Europe. Natural phenomena, such as volcano eruptions, soil erosion, sea salt and windblown desert sand can also cause air pollution.

We will never be able to eliminate air pollution entirely. The part of air pollution which we can do something about, is caused by human activities

Although the air quality has improved considerably over the last decennia, we still have a long way to go.

What kinds of pollutants are there?

When you breathe, you also inhale the substances polluting the air. Air pollution can therefore lead to health problems. Air pollution is a complex mixture of many substances. In legislation and research is often looked at specific substances. The most important substances are: particulate matter, nitrogen dioxide, ozone and soot.

Particulates are the most important constituent of air pollution for our health.

Click here to find out more about the different pollutants.

Is there always air pollution and is it everywhere?

The place where you are and the presence of sources such as traffic, factories or a large fire, determine the local air quality. In addition, the local air quality also changes with the passage of time. This is largely due to the weather. It depends on the weather whether the pollution will literally blow over or whether it will stay in the air. The supply of pollution from other areas depends on the weather as well.

Click here to find out more on the actual air quality.

Which factors affect air quality?


The amount of pollutants may vary from place to place, even within one and the same city.

After emission, the pollutants spread out in the air. Depending on the substances, they can either remain close to the source or be widely distributed.

The concentration of any substance will be highest close to the source. In general, small particles spread further from the source than larger particles. The smallest particles (ultrafine particles), however, will clump together into larger particles and are therefore mainly found near the source.

Thus, you will find ultrafine particles for example near roads with heavy traffic. The larger particles of generated by traffic will be found several kilometres away.


The amounts of polluting substances will change over time as well. Aside from the weather conditions, at times when traffic is busy, such as the rush hours in the morning (between 7:00 am and 10:00 am) and evening (between 5:00 pm and 08:00 pm) the concentration of air pollutants will be at its highest. In addition, there will be differences from season to season as well (see ‘weather’).


Last but not least, the weather will have an influence on the polluting substances as well. Under certain weather conditions, the concentrations of pollutants can rise. This is the case when there’s no wind or when other certain specific weather conditions occur. During hot Summer days high ozone concentrations can also contribute to air pollution.

However, the wind can carry certain substances over long distances. This way the pollution generated in our country ends up in neighbouring countries as well. And vice versa. The distribution of these substances therefore demands a local and national as well as an  international approach.

Can you smell or see air pollution?

In general, you can neither see nor smell air pollution, unless the concentrations of some pollutants are extremely high (but this is very rare). On days with extreme concentrations of particulate matter you can often see a mist of swirling particles above cities, which is referred to as smog (a combination of the words 'smoke’ and ‘fog’). In Northwestern Europe, this is a rare occurrence, in Beijing in 2012 there was a period of very strong smog.

What is air pollution ? Both indoors and outdoors, we inhale polluted air without even being aware of it. Lisa points out when to be careful and pay attention.
Move the cursor to see the evolution of pollution according to the seasons

Lisa loves to jog in Spring when all the trees and flowers are back in bloom. She tries to avoid parks or woodlands near a busy road, because otherwise she might inhale traffic generated exhaust fumes.


During Summer holidays Lisa likes to run on the beach between the dunes, but she makes sure to never go running on hot days when there is a risk on high concentrations of ozone.


Camp is all about games and sports. After a long day, Lisa and her friends like to relax around the campfire. But they’re always very careful and try to limit their time by the campfire since it contains soot and other dangerous substances.


After going for a run, Lisa likes to reward Lisa herself with a cup of hot chocolate by the fireplace. By burning wood however dust is generated, not only outdoors but indoors as well. That’s why it’s important to ventilate your house, even in Winter!

More info

What is particulate matter and what are ultrafine particles?

Particulate matter is the collective name for all particles invisible to the naked eye that are suspended in the air, such as soot particles. These particles vary in their origin, size and composition. Particulate matter constitutes the element of air pollution that causes the most health problems. The smaller the particles, the deeper they penetrate into the lungs and the more damage they can cause.

Particulate matter can be defined as:

  • PM10: ‘large’ particulates. These are particles with a diameter of less than 10 micrometres* (µm). A human hair is 5 times as thick.
  • PM2,5: ‘fine’ particles, with a diameter of less than 2.5 µm. This categoryconsists primarily of particles created by combustion processes, such as soot particles from diesel engines.
  • PM1: particles with a diameter of less than 1 µm.
  • P M0,1: ultrafine particles. These are particles with a diameter of less than 0.1 µm.
  • *1 μm = 0,001 mm.

What is nitrogen dioxide (NO2)?

Nitrogen dioxide is the gas released in combustion. In the outside air, traffic is the major source of nitrogen dioxide. Nitrogen dioxide is a substance that can easily be measured. If the nitrogen dioxide concentration is high then, often, the concentration of other traffic emissions is also high. Nitrogen dioxide is therefore often used as a gauge for air pollution caused by traffic. It is a ‘traffic indicator’.

What is soot?

Soot consists of tiny particles produced in combustion processes. It is also sometimes referred to as black smoke, or elementary carbon. Soot constitutes part of the particulate matter, but is can also be measured separately. It is a better gauge of traffic-related air pollution and, according to the latest insight, it can be a better indicator of health effects.

What is ozone  (O3)?

Ozone is a gas. It is formed from nitrogen oxides and hydrocarbons, under the influence of sunlight. The ozone concentration is highly influenced by the weather. You find particularly high concentrations of ozone in the summer, on sunny, calm days with an east wind, especially in the afternoon and early evening. The amount of ozone increases during the day and is at its highest between 12:00 pm and 08:00 pm. At night, the ozone at ground level is largely broken down.