Because breath is usually invisible, we think of it as immaterial or hovering on the edge of materiality. Cold: breath becomes water vapour as it leaves the mouth. With this slight incarnation of the breath, children become pretend smokers, and sometimes we feel our animality a little closer, becoming aware of our breathing and its rhythm, of the cold air drawn into our lungs and of its dispersal in the atmosphere. Much colder: the breath freezes as soon as it leaves the mouth, becoming thousands of tiny ice crystals which fall tinkling to the ground. The Russians call this "angel dust".

Not so long ago, easily within living memory, London was the host to smogs so thick that people lost their way even in the streets in which they lived. One bout killed thousands. Now the deadly air is mostly invisible, although on some days it smothers the city like a soiled blanket. But more and more children labour with asthma while adult hearts burst with the strain of breathing machine exhaust. Take the tube or the train out to one of London's northern hills, or further out into the country where the air, at least away from the roads, is clear, where trees expand in it and perfume it. There it becomes obvious that air, as much as light or space, is a commodity. Some people buy it (as part of a quality of life package perhaps) while others are forced to breathe heavily, uncertainly, and eventually, often sooner than the buyers of clean air and effortless respiration, not at all.

Those whose breath does not come easily, whose airways rebel against the atmosphere, become measuring instruments of this change in air quality from place to place, even from one side of the street to another (pollutants in their cycle between buildings tend to linger on the leeward side). Levels of contamination are registered with coughs, wheezes and splutters. So these people, ever more numerous, walking the districts of the city, are learning about the commodification of air.

Apparent nothingness, invisibility, imperceivability is the perfect state of breath. Passion may constrict the airways and halt speech. In computer-mediated communication we take on some of the machine's perfect progress; confidences, anger, the expression of love, come easier because we are not impeded by our bodies and our breath. Identity can be constructed with slow deliberation, almost as a work of art is constructed. Yet with this use of the universal simulator the capacity for dissimulation also increases. The increase in power and control that this disembodiment involves - the advantages of role playing, the blindness of those you "speak" with to your disability, disadvantage and other attributes which inspire prejudice - is bought at the price of making each interlocutor a little less than human. Face-to-face, we hear the breath in the voice, and gain a feeling, perhaps specious, of the whole presence