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Clouds & Particles

Particles and respiratory tract

Particles are dangerous for the health. The toxicity of particles varies with their chemical composition, and their size: the finest the particle is, the deeper in the lung it will penetrate. Scientists have therefore decided to classify the particles into two size ranges, called PM10 and PM2.5.

“PM” means Particulate Matter, that is another name for particles. PM10 means Particles that have a size less than 10 microns; PM2.5 means that the particles size is less than 2.5 µm. The size of airborne particles is significant, as this determines in which parts of the respiratory tract the particles are deposited, as well as how rapidly and the manner in which they are cleared.

1. Respiratory tract. Click to enlarge! (68 K).

The respiratory system

Look on the left at the respiratory tract, and the penetration of particles according to their sizes.

1: Pharynx
2: Larynx
3: Trachea
4: Bronchus
5: Bronchioles
6: Pulmonary Alveoli

You can see that the coarsest particles (from 3 to 10 micrometers in diameter ) tend to be deposited in the upper parts of the respiratory system, from which they can be eventually expelled back into the throat.

Between the two, PM_2.5 are responsible for causing the greatest harm to human health because they are so small. These fine particles can be inhaled deep into the lungs, reaching the 600 millions of pulmonary alveoli. They can cause breathing and respiratory symptoms, irritation, inflammation and cancer.

Smallest particles

Scientists attach nowadays more and more importance to the smallest particles, which are of 100 nanometres in size, because their size are in the same order of magnitude as the ADN molecule (2.5 nm) and the human red corpuscle (800 nm). They can reach the deepest parts of lung area and even go into the blood circulation, leading to cardio-vascular disease.

2. Red corpuscules. Source:http://www.ulb.ac.be/sciences/biodic

Chemical composition

Chemical composition of particles accounts for a very large part of their toxicity. The composition may determine in what way the respiratory tract reacts, or the body responds. The toxic air pollutants attach themselves to particulate matter that floats in the air. Some particles can act as carriers of hazardous chemicals, like pesticides, heavy metals, toxins… These toxics are then delivered into the lungs, where they can be absorbed into the blood and tissue.

3. For sampling of particles, air is pumped through a filter, on which the particules deposit. On the left, a new filter. On the right, a filter on wich particles were collected. Look at the colour difference! Source: I. Cousteix.

Pollution is a strong source of particles, especially very small ones, which pose the greatest hazard to health. Particles emitted from diesel vehicles are smaller than particles from petrol cars, even though the particulate emissions from diesel engines have already been reduced in recent years, for example by adding pipe filters.

About this page...
Author: J. Gourdeau, LAMP Clermont-ferrand, France
scientific reviewing: Dr Karine Sellegri, LaMP, France.
Date of generation:2003-10-23.Last published: 2004.13.05.