The air doesn't know about zoning boundaries
Work uses suggest another bugaboo: reeking smokestacks and flying ash. Of course reeking smokestacks and flying ash are harmful, but it does not follow that intensive city manufacturing (most of which produces no such nasty by-products) or other work uses must be segregated from dwellings. Indeed, the notion that reek or fumes are to be combated by zoning and land-sorting classifications at all is ridiculous. The air doesn’t know about zoning boundaries. Regulations specifically aimed at the smoke or the reek itself are to the point.