It is not generally known that certain poisonous substances are legally allowed in commercial spirits.
Home distillers are concerned about poisonous substances such as methanol, arsenic and other nasty stuff. However, have a look at the LEGALLY allowed levels of nasty substances in wine and spirits.
The above is applicable in South Africa (June 2010). The full regulations are available at: http://www.sawis.co.za/winelaw/download/Regulations,_annotated_06_2010.doc
It is rather interesting that responsible home distillers are not allowed to sell their products, even if their products contains much less harmful substances than commercially available "legal" spirits!
It should however also be noted that, although these substances are highly poisonous in high concentrations, the above limits are considered safe for human consumption.