【Pernicious】 , 【baneful】 ,  【noxious】 ,  【deleterious】 ,  【detrimental】  are comparable when they mean exceedingly harmful but they differ as to the kind and extent of the potential for harm.

Something is either  【pernicious】  or  【baneful】  which is irreparably harmful but  【pernicious】  is more often applied to things that harm exceedingly or irreparably by evil or by insidious corrupting or enervating and  【baneful】 to those that poison or destroy.

Something is  【noxious】 which is harmful especially to health of body or mind.

【Deleterious】 is used chiefly of something which causes harm when taken into the body (as into the digestive or respiratory tract) and may suggest obscure or ill-understood effects.

【Detrimental】 , like  【deleterious】 , generally suggests a much lower degree of harmfulness than the remaining terms; typically it imputes an impairing or hampering quality to the agent or an impaired or hampered condition to the one acted upon.