As you are reading this article, the chance is pretty big that you are breathing right now. Without even noticing, you are taking in the air around you. That air (hopefully) doesn't smell, isn't colored and probably looks pretty empty. It is, however, full of different particles, invisible to the naked eye. These particles may be a sign of air pollution and they have increasingly been linked to several diseases, including asthma, COPD, cardiovascular disease and lung cancer. Recently, it has become clear that air pollution exposure is a risk factor for ILD as well.
Whether you live in a busy city or a rural area, the air around you is full of different gasses and particles. And even though we need the oxygen in the air to survive, this is not the most abundant component of air. It makes up about 21% of the normal air around you. By far the most abundant gas in the air is nitrogen with 78%. That leaves about 1% of the air that is made up of a lot of different gasses, like argon, carbon dioxide, neon, methane, helium, krypton, hydrogen, xenon, ozone and nitrogen dioxide. Air also contains a variable amount of water vapor, depending on the temperature of the air.
It is a complex mixture of substances suspended in the air in which different sizes are distinguished.
Apart from gasses, the air around you (or 'ambient air') also contains a variety of particles, floating around in the air, invisible to the naked eye. These particles can be of natural origin, like pollen, sea salt (when sea water evaporates) or sand dust (which can be blown by the wind from the Sahara all the way to the northern parts of Europe). But if you are living in an urban area, chances are that the air around you is affected by particles of unnatural origin, particles which are the result of human activities. This results in air pollution. Air pollution is composed of a mixture of different elements like particulate matter, nitrogen dioxide, sulfur dioxide, black smoke and ozone (see Box 1).1,2 Particulate matter, the ozone concentration at ground level and nitrogen dioxide are most strongly associated with lung diseases.3 According to dr. Frans de Jongh (lung physiologist and Assistant Professor at the University Twente, the Netherlands) particulate matter is especially damaging to the lungs. “It is a complex mixture of substances suspended in the air in which different sizes are distinguished. It contains all sorts of materials, like salt and sand, but also diesel exhaust particles or heavy metals. The smaller the particles, the deeper they can be inhaled and the bigger the health problems they can cause. Most damaging to your health are particles with a diameter of ten microns or less.” As you can see in Figure 1, ten microns is about five times smaller than a human hair!
“Air pollution in urban areas is caused by both mobile sources, like road and air traffic or stationary sources, like industries. Households contribute to air pollution as well, for instance burning wood for cooking and heating”, De Jongh explains. But this does not necessarily mean that living in a rural area, results in you breathing clean air. Particulate matter is also generated by intensive livestock farming, and can be brought in from countries surrounding yours.1 However, not all air pollution originates from human activity. As said earlier, in some countries, air quality is largely influenced by dust storms, particularly in regions close to deserts.4 Also the weather (and most importantly wind direction) has a big impact on air pollution. Depending on the direction, wind can blow air pollution out of or into certain areas. On the other hand, rain makes the particles precipitate, cleaning the air.
The World Health Organization (WHO) keeps a database in which they collect measurements of mainly urban air quality.4 They have calculated that 92% of the world’s population live in places with air pollution. In Box 2 you can find more information about how air quality is measured.
Cities in India, Saudi Arabia and China constitute most of the upper list of most polluted cities globally (see Box 3).6 However, not all cities collect information or record ambient air pollution, among them some cities which are suspected to be the most polluted.4 The areas with the cleanest air in the world are found in de United States of America and Northern Europe (see Box 4).6
You can check out the air quality in your own area by looking at the WHO interactive air pollution map: http://maps.who.int/airpollution/ (see also Figure 2).
The relationship between ambient air pollution and ILD has only recently become clearer. De Jongh: “It is now thought that air pollution plays a role in the development and aggravation of this disease.” Unfortunately, a lot is still unknown. He continues: “We do know that in ILD, the smaller particles are mainly responsible for damage to the lung tissue. Most of the PM10 particles never even make it into the lungs. With the speed they are inhaled, they ‘smash’ into the back of the throat, after which they are swallowed and end up in the stomach. Smaller particles, especially those of three microns and less, do reach the lungs where they may have a negative effect on the lung tissue.” But not everyone is affected in the same way by these particles. “In cleaning up the particles that get stuck into the tissue, a person’s genetic or immunological profile plays an important role. This means that some people are born with the capabilities of nicely cleaning up the particles that have been inhaled. The body breaks them down, after which they are eliminated. In others, this mechanism does not work as effectively. Unfortunately, we do not yet know who has this favorable genetic profile and who does not,” De Jongh says.
So, for now the most important way of reducing the effects of air pollution in ILD is to make sure you inhale as little of these damaging particles as possible. Moving to the parts of the world with little air pollution is not a very realistic option. However, most sources of outdoor air pollution are well beyond your control and demand action by cities, as well as policymakers in sectors like transport, energy waste management, buildings and agriculture.4 In lung diseases like asthma and COPD, it is quite clear how patients can minimize their exposure to irritating agents, like sanitizing their homes and quitting smoking. But in ILD this is less clear. De Jongh: “Keeping your home and work environment as free from polluting particles as possible is important. Using filters to clean the air will help, but their effect is limited as in ILD the smaller particles, not caught by these filters, play an important role.” But is the best way to avoid air pollution altogether staying inside? De Jongh does not think so: “I will not advise ILD patients to stay inside. When you go outside and exercise for example, you will indeed inhale more particles. But the health effects of exercising outweigh the negative effects of staying inside and doing nothing.”