Although light and air are commonly coupled together in everyday language, when speaking of easements in connection with buildings it is necessary to distinguish between them, and to take note of some very important differences in the law applicable to each.

Air, like light, is one of the necessaries of life, and all people are entitled freely to use and enjoy it in a reasonable manner ; but people may not use it so as illegally to injure others, nor may they illegally prevent other people from having such reasonable use and enjoyment of air as they may be entitled to.

It follows that there must be easements relating to air, but they differ from light easements as widely as air differs from light.

While light, with its attendant radiant heat, is motion, and has no mass or substance, but is a vivifying, stimulating, and purifying influence which travels mostly in straight lines, and can freely penetrate closed glazed windows, and is for the most part quenched in the uses to which it is put ; air is a gaseous vaporous mixture of very composite constitution. It has substance, weight, and volume, and it moves bodily from place to place by gaseous diffusion or intermolecular expansion in all directions, and sometimes by flow in particular directions as air streams or currents. These air currents have not, as a rule, any sharply defined borders, unless they are enclosed in pipes or other channels bordered by walls. They do not ordinarily travel far in definite straight lines, as light does.

Also, air has not always the same composition. It can pick up and incorporate with itself by gaseous diffusion many gases and vapours injurious to life and health, and it can carry in suspension particles of dust and dirt, and the eggs or germs of large numbers of minute animal and vegetable organisms, many of which may be highly inimical to the health and life of man.

Also, air can only pass into or out of buildings through open apertures. The same windows which will admit light and air when open will admit only light and exclude air when closed, and a screen which may effectively stop the access of nearly all light to a window may not prevent the access of a plentiful supply of air.

The laws affecting the use and enjoyment of air must necessarily depend largely upon the natural physical and physiological properties of the substance concerned.

In former days, when less thought and attention was given to ventilation of buildings and to the effect of atmospheric surroundings upon the health than in these, there were comparatively few private disputes about air which came into the law courts for adjustment, and the total number of reported cases in which easement rights relating to air exclusively have been tried and decided is still very small by comparison with cases relating to light or light and air together. It seems probable that in proportion as people learn to set more and more value upon good ventilation and purity of atmosphere, disputes about air are likely to be more frequent than they have been in the past.

Easements in connection with air may have reference to access, exit, and route by which air should be allowed to travel, and they may also have reference to the purity or pollution of the air. Ordinarily all owners or occupiers have similar and equal rights as regards the uses they may make of their land or houses or other buildings, unless it so happens that special privileges have been acquired for, or by, the owners of particular tenements, by the exercise of which privileges the owners of neighbouring tenements may be lawfully compelled to put up with some restriction of their natural legal rights, or some injury or discomfort for the benefit of the dominant owner who is privileged.

The dominant owner is privileged to do something in excess of his ordinary natural rights, which affect his neighbours in the use and enjoyment of the natural rights appertaining to their tenements, such as they, in their turn, could rightly complain of or suppress as a trespass or a nuisance but for the existence of the privilege. In the case of Hall v. The Lichfield Brewery Company, decided in 1880, the plaintiff, who was a butcher, owned a slaughter-house with apertures for ventilation. There had been no material change in the structure of the slaughter-house, which had been used in its present form for more than thirty years before suit. Defendants built on their land so as to obstruct the free flow of air to and from the apertures. Plaintiff recovered damages for the obstruction on the ground of an implied covenant not to interrupt the free access of air suitable for the purpose of a slaughter-house.

In this and other cases relating to access or exit of air there have been definite apertures which have been more or less obstructed when the plaintiff has succeeded, but it has been held that in the case of a windmill there is no cause of action to restrain a neighbour from building on his land, although the building will interfere with the free flow of air in a certain direction towards the sails of the mill. Similarly it has been held that building in such fashion as to interrupt a free flow of air towards a neighbour's chimney pots gives no cause of action, even when the owner of the chimneys is able to show that his chimneys smoke in consequence.

A very interesting case relating exclusively to air was that of Bass v. Gregory, where the plaintiff claimed to possess an easement right for passage of air through a defined channel, and to discharge foul air coming from his cellar into an old disused well in the yard of a neighbouring owner, who claimed a right to obstruct the orifice.

The chief facts of the case were these. Plaintiff owned a public-house, under which was a cellar, which was ventilated by a shaft cut therefrom through the solid rock, and terminating in an old well in a yard occupied by the defendant. The cellar was a very ancient one, excavated from the rock, and had been formerly used for brewing, for which use ventilation was necessary, and it was found, as a fact, that the ventilating shaft in question had existed certainly for more than forty years, and that there had been communication through that shaft to the well and up through the well and the grating at the top of it into the open air, so that the well became the ventilating shaft for the cellar. It was also found that this state of things was known to the defendant, and to those before him who occupied the yard in which the old well was.