Zoroastrian law forbids both burial and cremation, and it forbids them for the same reason it forbids almost everything else concerning a corpse. Earth, fire and water are pure. A dead body is the most polluting thing there is. It may not be given to any of them.
The traditional solution was exposure. The body is carried to a dakhma, a stone tower open to the sky, and left for birds to strip; the dry bones are then placed in a central ossuary pit where lime and rain reduce them. The system is efficient, it uses no fuel and no land, and it depends absolutely on there being vultures.
There are almost none left. Populations of the resident Gyps vultures across the Indian subcontinent collapsed by more than ninety-five per cent from the mid-1990s. The cause was identified in 2004: diclofenac, an anti-inflammatory given to cattle, which is fatal to vultures that feed on treated carcasses. India banned the veterinary formulation in 2006. The birds have not returned at anything like the rate that would matter to a community with a functioning dakhma.
The consequence at Doongerwadi, the wooded Parsi funerary estate in central Mumbai, is that bodies which once disappeared within hours now do not. The Bombay Parsi Punchayet installed solar concentrators, arrays of mirrors that focus sunlight into the tower to desiccate the remains, on the reasoning that sunlight is not one of the prohibited elements. They work in the dry season and much less well in the monsoon. Some Parsis have moved to burial or to cremation, which the orthodox regard as a straightforward abandonment of the law; others regard the concentrators as an equally objectionable innovation; a vulture breeding programme has been discussed for years.
This is worth stating plainly because it is an unusual kind of religious problem. Nothing about the doctrine has changed and no one has argued against it. A veterinary drug removed a bird, and a rite that had worked for something like a thousand years stopped working. Zoroastrians in Iran had already largely moved to burial in concrete-lined graves during the twentieth century, on the reasoning that concrete keeps the corpse from touching the earth, and that solution is now discussed in India too.