The reason for our exposure to air pollution on a daily basis is that air pollution moves easily and quickly through the air. Pollution from vehicle exhausts or industrial plants for example can easily spread out over miles. Air pollution in the developed world is mostly invisible, so you can't see it moving and you can not visibly tell that you are being exposed to it. The movement of pollution over short and long distances is complicated and still the subject of research. However, there are some basic scientific laws which govern the movement and our exposure to air pollution.
Air pollution, whether indoors or out, is either particulate or gaseous in nature. Particulate pollution is often classed according to its size and known as particulate matter (PM). Therefore, PM10 refers to particles of size 10 microns or less, while PM2.5s are those of size 2.5 microns or less. Meanwhile, ultrafine particles have a size of 0.1 microns (100 nanometres) or less. Particles can also be described according to their source, such as pollen grains, bacteria, mould spores and house dust mite droppings. Gaseous particles are on a molecular scale and a molecule is much smaller than a particle, being on the nanometre scale.
The gaseous molecules which form the main components of our exposure to air pollution indoors and outdoors include:
Larger particles, like pollen grains or bits of dust, are carried around on thermal air currents (think of the dancing dust particles you see in a beam of light). Eventually these larger particles will settle onto a surface, under the influence of gravity.
Smaller particles are subject to a process called Brownian motion, which was discovered by Robert Brown, the English botanist, in 1827. In Brownian motion, a molecule, or bigger particle, which is suspended in a liquid or gas, will constantly be buffeted by the similar motion of neighbouring molecules, keeping it into motion in a random way which propels it through the air.
Our exposure to air pollution from gases are subject to the laws of diffusion and effusion. These laws were formulated by the Scottish chemist Thomas Graham in 1832. Diffusion is the gradual mixing of molecules of one gas with those of another gas, owing to their constant motion (as described above). An obvious example is when someone sprays herself with perfume at the other end of the room, you will smell it within a few seconds as the molecules travel into your vicinity so they can be inhaled. If you suffer from Multiple Chemical Sensitivity, you may find yourself experiencing a reaction to the perfume such as a headache, or sneezing. You can choose to go outside for some fresh air. What is more dangerous, however, is the diffusion of toxic carbon monoxide molecules from a faulty boiler to be inhaled, day after day, by the unsuspecting occupants of a poorly maintained property. Similarly, our exposure to air pollution due to household products like carpets and furniture may emit formaldehyde into the surrounding air, where it diffuses towards the room's occupants. This outgassing, as it is known, may continue for weeks or even months. It is best avoided by trying to avoid products that off-gas (check the label for low, or no, formaldehyde and VOC products) or by using an air purifier forgas and chemical pollution with high quality granular activated carbon.
According to Graham's law of diffusion, our exposure to air pollution due to the rate of diffusion is inversely proportional to the square root of the molecular mass of the gas. Put very simply, the heavier the gas molecule, the slower it diffuses through the air. Common pollutants, as listed above, tend to have fairly similar molecular weights so will diffuse at similar rates. The best approach to avoid inhaling them is to use an air purifier with a gas absorbing media and open the windows for ventilation as often as you can.