No person, firm or corporation shall hereafter engage or continue in the business of pest control within the City by means of generating, releasing or spraying poisonous or dangerous fumes, gases or vapors, such as hydrocyanic acid, cyanogen, salts of cyanides, methyl bromide, carbon dioxide, carbon bisulfide, carbon tetrachloride, chloropicrin, ethylene dichloride, ethylene oxide, methyl formate or other poisonous substances or mixtures of these compounds, in a building or part thereof or in a closed space, vessel or structure within the City for the purpose of fumigating the same or exterminating insects, vermin, rodents or other pests without first obtaining a license therefor from the Director of Licenses and Permits. Such license shall be issued only upon application to the Director, after approval thereof by the Commissioners of Inspections and Licenses and Fire and after examination of the applicant as hereinafter provided.