All thermal processes create emissions, but the plant cleans them before releasing them to the atmosphere so they are not harmful.  This process is controlled strictly by the regulations and the local authorities and the Environment Agency in England and Wales, and SEPA in Scotland monitor all energy from waste plants to ensure they do not emit substances in quantities harmful to human health.  

Before a plant can be operated, it has to obtain an environmental permit which states the maximum level of emissions the plant can emit.  If the plant exceeds any of the limits it must report it to the regulatory authority and take corrective action. If it fails to satisfy the authorities, the plant will be closed down.