Three potential sources of pollution
, with very different behaviours and lifetimes. Pollutants are either: persistant but predictable or inpermanent and unpredictable.
Radioactive fallout produces various metal ions that are chemically and biologically identical to their non-radioactive cousins. Although non-toxic, and with no effect on metabolism, they are genetically dangerous. Mutations caused by radiation accumulate and can cause cancer in high doses ("1mSv [sievert = Joule per kilogram] per individual averaged over a whole population is believed to result in an average total of 13 cancers and 8 genetic defects per million of the population" - Book of Data). Since the ions are not broken down, radioactive agents are persistent - even though some have short half lives, Uranium's is 7 times 108 years.
This type of pollutant can be toxic to some organisms and not to others (very few creatures are radiation resistant). Chemicals can be converted to toxins, however, and concentrated by fatty tissue up the food chain. Concentration is important, since low levels can be tolerated, but the top consumer may recieve a lethal dose.
Below the polluting concentration, the chemical might not be dangerous. Their persistency is variable, therefore, as is the predictability of the results of release into the environment.
DNA is not noticably toxic (obviously), but could code for toxic proteins (the Bt toxin for example) or toxin producing enzymes. Additionally, higher order interactions between a gene product and the existing proteins in the cell are possible. Aggregation (as in alzheimers), suppression of essential genes or interference with DNA replication would all be deadly. OTOH, these effects are not transmitted as easily (although prions can be) as chemical or nuclear agents. The worst case scenario would be viral transmission (transcapsidation), since a virus could spread the gene from organism to organism, essentially like a chemical agent. Like chemicals, gene products can be toxic - some claim that they can be allergenic (like some pesticides, no doubt). Also, again like pesticides, GMO crops producing toxic proteins could encourage resistant pests.