Waste-to-Energy Plants are strictly controlled. They comply with the most stringent emission limit values applied to any single industry, set out in the Waste Incineration Directive 2000/76/EC.

While dioxins exist naturally in the environment the manmade ones come from a variety of combustion processes including steel mills, power plants, cement kilns, diesel vehicles, buses, open fires in the home, bonfires, barbeques, jet engines, forest fires...
Emissions from Waste-to-Energy plants present just a tiny fraction of such emissions.

Whereas in 1990 one third of all dioxin emissions in Germany came from Waste-to-Energy Plants, for the year 2000 the figure was less than 1%. 

A recent study carried out by Lisbon University’s Institute of Preventative Medicine calculated that waste incineration “does not impact on dioxin blood levels of nearby residents” to Waste-to-Energy Plants (http://www.sciencedirect.com/).

The UK Committee of Carcinogenity found that “any potential risk of cancer due to residency near to municipal solid waste incinerators was exceedingly low and probably not measurable by the most modern epidemiological techniques” http://www.advisorybodies.doh.gov.uk/Coc/munipwst.htm 

The Scientific Advisory Council of the Federal Medical Association Germany, investigated potential health risks caused by emissions of Waste-to-Energy Plants, concluding: “The evaluation conducted shows that currently operating Waste-to-Energy Plants, which are conform to the technical standards, cause very marginal health risks which can therefore be classified as negligible health risks for the population living in the vicinity of Waste-to-Energy Plants” Source: German Medical Journal 90, edition 1 / 2, 11th of January 1993,  p. 45-53, Publications