Do the Zoroastrians dispose of their dead in the towers of silence because of a shortage of burial space?

Formally, Zoroastrians dispose of their dead in the Towers of Silence for religious reasons. They believe that the dead body is unclean and to bury it would pollute the earth. Similarly, cremation might pollute the fire or air. Placing the bodies up  high where they are exposed to the sun and carrion birds allows for the remains to be disposed of or "recycled" in a way that prevents the spiritual pollution of the environment. 

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Formally, Zoroastrians dispose of their dead in the Towers of Silence for religious reasons. They believe that the dead body is unclean and to bury it would pollute the earth. Similarly, cremation might pollute the fire or air. Placing the bodies up  high where they are exposed to the sun and carrion birds allows for the remains to be disposed of or "recycled" in a way that prevents the spiritual pollution of the environment. 

This means of disposing of the dead has been practiced for at least three thousand years. However, it is predated by the practices of cremation and embalming before burial. Because of this, one might assume that the practice arose as a response to a lack of space for interment. That would make sense from a structural-functionalist point of view, and it is a possibility. It is also possible that there were negative consequences, such as spread of disease, which resulted from burial in the ground or in tombs. This desire to prevent the spread of disease or avoid the smell of decomposition is at the root of most burial traditions. Open-air burial is just the way that Zoroastrian people prefer to do it, believing it is the best possible means of disposal of the dead.