In the 1970 case Johnson v. United States, the Supreme Court ruled that a warrantless search of an apartment was not justified by the smell of burning opium alone. The police had entered without a warrant after smelling what they believed to be burning opium coming from inside and arrested Homer Johnson for possession of drugs. The court held that "mere evidence" did not meet the criteria necessary for a warrantless search under exigent circumstances exception to Fourth Amendment protections against unreasonable searches and seizures. This decision reaffirmed citizens' rights to privacy in their homes, emphasizing that law enforcement must have more than just suspicion or potential evidence of criminal activity before infrallying on these rights.

In the dissenting opinion for Johnson v. United States, Justice Harlan argued that the majority's decision to suppress evidence obtained through a warrantless search was incorrect. He contended that there were exigent circumstances which justified immediate police action without obtaining a warrant first. In this case, law enforcement officers smelled burning opium from outside an apartment and entered immediately out of concern that evidence might be destroyed if they waited for a warrant. Justice Harlan believed these conditions met the criteria for an exception to the Fourth Amendment's requirement of obtaining a search warrant before conducting searches or seizures in private residences. He also expressed concern about potential negative impacts on effective law enforcement due to overly strict interpretations of Fourth Amendment protections against unreasonable searches and seizures.