Clean air does not mean air devoid of everything other than molecules of nitrogen, oxygen, carbon dioxide etc. The Pleistocene air our Palaeolithic ancestors breathed contained micro-organisms (bacteria , fungal spores), water vapour, natural volatile organic compounds given off by plants and other life (and no toxic VOCs given off by plastics, fertilizers, fossil fuel combustion etc.). The same air immersed all other life on the planet: plant, fungi and animal.
During the millions of years of human evolution our ancestors co-evolved with the air of temperate rainforest environments and its health-promoting effects are one of the most neglected by scientists in our increasingly prosthetic world.
150 years ago the most effective treatment for tuberculosis was to go to a place in the country where one would sit outside in the sun – its health-giving effects were not just the leaving behind of indoor, urban air, but also the immersion in natural air and inhaling it with every breath. Human lungs contain about 600 million alveoli with a total surface area of over 100 sq metres; we need to be more conscious of what we should allow to touch this relatively large and sensitive interface we have with the rest of the environment. 
Volatile signals and triggers
These have profound significance. Few of us comprehend the degree to which most lifeforms and much of bio-chemistry (including that of Homo sapiens) is influenced by volatile signals and triggers. It follows that how we pollute our air affects these signals and has far more than their direct chemical effects.
Much of the ecology of micro-organisms, insects and plants is governed by bacteriostatic, fungistatic and hormonal volatiles that influence the induction, growth, death, sexual and other behaviours of those organisms. These volatiles can be effective at parts per billion concentration. There is no reason to assume that human bio-chemistry is any different except that we can overide our responses to these natural signals via our brains.
For example, apricots are often infected by spores of Sclerotinia fructicola  at the flower stage with these spores remaining dormant for months untill volatile aldehydes and ketones are produced at minute concentrations as the fruit ripens. These volatiles switch on pectinase and polphenol oxidase enzymes in the spores that make them into lethal brown rot: first parasites and then pathogens. Volatiles from the rotting tissue then induce more spore formation which then overwinter but are reactivated by seasonal volatile and temperature signals to repeat the cycle.
Similarly with humans we are a zoo of organisms and many latent infections that may similarly be repressed or switched on by changes in our physiology (fitness) or external environment (eg allergens, carcinogens, pollutants). There are many diseases associated with such volatile environmental triggers. Most antibiotics, herbs and fragrances work similarly via volatiles that switch on different physiological or neurological processes. Sex, fear, rage, calm, alertness and excitement are all variously governed by these volatile signals.
So what is the value of clean healthy air? No less than your mind, your health, your life.
1. Renamed Monilinia fructicola during the twentieth century
2. The New York Times reported on 15 September 2009 that "There are some things it is better just not to think about. Like the 10,000 bacteria you inhale with each breath in the average office building. Or the 10 million bacteria in each glass of tap water. Microbiologists have now added something else to the list of things too gross to contemplate: the deluge of bacteria that hit your face and flow deep into your lungs in the morning shower. Showers in New York carry a particularly high dose of a microbe related to tuberculosis called Mycobacterium avium. The bacterium and its close cousins can cause a variety of exotic chest complaints, including lifeguard’s lung, hot tub lung and Lady Windermere’s syndrome. ... in showers bacteria are incorporated into fine droplets that can be breathed deep into the lungs." Back to text
3. Pharmaceutical companies have used the large lung surface area to enable speedy access into the body of their compounds, primarily using nasal sprays. Recent research (see BBC report) showed how the behaviour of men could be made more sensitive to other people by increasing their levels of the hormone oxytocin administered by a nasal spray. Back to text
Page established 3 June 2009 Page updated 1 May 2010