Legal requirements regarding 100% asbestos duct work
If you're willing to trust internet sources, a landlord is not necessarily required to remove hazardous asbestos, but the asbestos may be cause to break your lease without penalty or possibly even live there rent free until he remediates the hazardous asbestos (the asbestos in your house may or may not be classified as 'hazardous' depending on many factors such as whether or not it's in the duct seams or just used outside the ductwork as insulation in an inaccessible space - if he disagrees with your determination about whether or not it's hazardous it may take a lawyer and a report from an expert (or experts) to come to an agreement). 

Of course, if the asbestos is airborne (or likely to become so if disturbed), you probably don't want to live there for long  regardless of the landlord's responsibilities. If it were locked up in a painted over popcorn ceiling or in vinyl floor tiles that's one thing, but if you've got air blowing through friable asbestos in the duct seams, I wouldn't want to live in that house until it was professionally removed (and maybe not even then since the fibers will be in every nook and cranny of the house).

The landlord was probably required to disclose the existence of asbestos to you before you moved in -- and he may have already done so in some clause of the lease.

If conditions allowed you to be exposed to airborne asbestos that the landlord knew (or should have known) about, then he may have some additional liability for your exposure to it.

But, as @Grant said above, this question is what lawyers are for so you should consult with a lawyer for advice that applies to your situation. And of course, specific local laws may apply.