Roads have been reopened in Leederville after emergency services safely disposed of a concentrated ammonia solution found in Newcastle Street.
Two gardeners discovered a package containing the solution against the rear fence behind Leader Business Park and were overcome by fumes.
Newcastle Street was closed in both directions between Loftus Street and Oxford Street as four police in splash suits and breathing apparatus disposed of the ammonia.
It was diluted with water until the solution was harmless.
Detective Senior Constable Ben Lawson said it was unlikely the ammonia was used in the manufacture of illicit drugs but police would be investigating the source.
"Generally with ammonia it would be in a particular vessel...a jerry can with tubing coming off," he said.
Det. Sen. Const. Lawson said ammonia solution was quite dangerous and could be fatal in significant dose.
"It was a strong ammonia solution," he said.
"These guys deal with ammonia a fair bit and they were saying it was really giving off strong readings."