Do we have any way to break our lease since the landlord has not in fact fixed the issue and we continue to have problems?

My husband and I rent an apartment in Denver and have had recent problems with the neighbors below us. They smoke marijuana frequently and it's leaking into our apartment, causing headaches and a constant odor saturating our belongings. We spoke with the rental office, and they said they don't allow smoking and would deal with it via written notice. After reading the lease, I see that there is no mention of smoking in the apartments. We asked to be let out of our lease early because this is causing problems for us and the property manager refused. We also continue to smell it frequently, despite the assurance by the tenants below us that they have stopped.