With the recent realisation that mercury can transform itself in the stratosphere, then pollute the atmosphere below, we are finally coming to terms with the enormous extent to which we have (along with other sources) polluted ourselves out of house and home-planet. The extent of air pollution of course combines with terrestrial and aquatic pollutions, and even on to solar system and universal pollution. Of course it can't continue. If we ourselves are to stop it, government education would and could help a little.

The two main gases in our air are not pollutants at this time. Others are in small quantities and pose little threat until they upset ecosystems. The prime candidates are carbon dioxide, ozone and methane, although the latter is not sourced by humans, but instead lies beneath oceans as a rather benign, but massive threat. The many polluting "colleagues" of these gases are present in lesser quantities. We all know that some of them are lethal to us and other organisms. Carbon monoxide poisoning (mentioned later in car emissions) and the gases that governments have licensed so that we could harm each other are major threats despite their small volumes.

Global warming of course is the main threat from CO₂ build-up, as well as methane and several others. Gases such as these and even nitrogen can kill oxygen breathers by blanketing a low-lying area (such as a camp site). Creating world-wide nuisance however is their claim to present notoriety, despite several coal-burning countries' efforts to pour scorn on their effects. It is however accepted, both scientifically and rationally, that, whatever your reigning ruler might say, global warming is just about to cause a greater menace than we have faced for generations.

Vehicle emissions via Shutterstock

Carbon dioxide and several other gases cause a blanket effect on the earth. If we use a CO₂ fire extinguisher, it blankets the flames, robbing them of vital oxygen. In the case of the atmosphere, the same gas allows heat entry but prevents the loss of heat that would otherwise be reflected from the earth back into space. Strangely, sulphur dioxide allows more heat to escape and therefore its prevalence in past pollutions could have helped to prevent global warming!

Pollution from carbon dioxide results from cars and other vehicles (hence the proscribing of high emissions), fossil fuel burning (largely in power stations) and from respiration, particularly bacterial. By far the major cause currently is Chinese, Indian and US power stations. Partly from international cooperation in this last case and also because we have no alternative, most countries signed up in 1992 to a great international agreement called the Kyoto Protocol. This is now outdated and it is hoped that soon, we can concoct a more concrete agreement, especially from those countries who failed to measure up at Kyoto. The follow-up conferences in Bali, Cancun, Copenhagen and last December in Durban did little to stir the major defaulters into action, but have prepared developing countries for their huge responsibility to cut back on CO₂ emissions, just as their economies are growing.

Pollution from sulphur dioxide (and several other sulphurous gases) has also been traced to power stations, but such gases have been removed from emissions long ago in most cases. This is a classic case. Sweden began complaining to the UK and others 60 years ago that their forest loss and several other problems seemed to be caused by industrial pollution from British factories. It turned out that acid rain was caused by SO₂ in the local atmosphere and that similar problems caused Canadian effects from US sources. The Black Forest in Germany was threatened for a time by the emissions from many countries to the windward. Buildings, lakes and trees, along with fish suffering from watery derivatives, were the main victims of a severe acidic reaction. Nitrogen dioxide has a similar history to SO₂, as it also results from combustion. Cars emit a lot of NO₂, along with the infamous carbon monoxide. The demise of cars built as we know them at the present time could lead to a great decrease in some of these major pollutants, alongside the carbon dioxide we already try to limit with emission controls in new cars.

Air pollution in a city via Shutterstock

However lead and one or two other heavy metals have been carried by airborne pollution in the past and no doubt pose a threat alongside cadmium and several others at the present. Also present in the air are other solid particles, carbon being major among them. The health of humans is directly affected by such particles, but our remit here is less noticeable events. Photosynthesis is a feature process of all of our green plants we don't eat without photosynthesis. If masked by carbon or other deposits, plant leaves cannot operate and produce no carbohydrate. That means death, as it did when asteroids almost cut off this lifeline for years at a time in the past. Huge extinctions are a feature of the Earth's past, and this kind of air pollution seems mainly responsible. The Sun was blocked out, which was a major cause for deaths, but deposits on leaves could equally have caused a food chain failure when deposits covered the sky and the ground.

It is perhaps at the finale that we should add another of the less obvious, more insidious air pollutants. The POPs are volatile but relatively insoluble, hitching a ride on solid particles to travel enormous distances to countries that have never produced them. They are of course the Organic Pollutants that Persist in the environment. Their main feature is their toxicity, with even some carcinogenic properties, but there are a vast range of them from PCBs to Agent Orange. Their effects may still be very much in the environment but awareness of their presence in animal fats (many being fat soluble) is leading to a small decrease in their known effects on humans.

Other air pollution events have included CFCs (chlorofluorocarbons). They were used in fridges, but discarded when the ozone in the atmosphere was destroyed by them. Ozone is strategic in preventing ultraviolet light from entering the atmosphere and causing big increases in mutation rates. The legacy of the CFC pollution is our commitment to watching the two ozone holes at the Earth's poles, in case any gases start affecting their size again! There are many such candidates in air pollution.