Therefore air has no Value. Exchange-Value is a superficial phenomenon, is a ratio between any two commodities. It but externally gives expression to the inherent property of the separate commodities, the fixed or embodied Labour or Value. Substances without Value have no Exchange Value, e.g. air. In Value there is no substance and therefore no Utility, only Labour, and that only as found absorbed by substance.
Labour spent is measurable by time.
It is concrete when we think of the particular commodity produced. It is abstract when we think of the individual's loss of energy. Concrete - Bread Concrete labour produces useful articles, Abstract labour produces Value. The same person can do various types of work. Therefore there is no inseparable gulf between one kind of labour and another.