FUMES IN THE CABIN AFTER TKOF LEAD TO A RETURN TO DEP ARPT. DISAGREEMENT BTWN CREW MEMBERS ABOUT DECLARING AN EMER.

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IMMEDIATELY AFTER TKOF FROM RWY 21L AT DTW METRO ARPT; THE LEAD FLT ATTENDANT CAME INTO THE COCKPIT IN AN URGENT MANNER AND INFORMED THE CAPT THAT THERE WERE STRONG FUMES COMING FROM THE REAR OF THE CABIN. THE CAPT STATED THAT HE COULD SMELL THEM AS WELL; (I COULD NOT SMELL THE FUMES) AND MADE AN IMMEDIATE DECISION TO RETURN FOR LNDG. AS WE ARE TRAINED AT OUR ACR; THE CAPT ASSUMED THE DUTIES OF HANDLING THE EMER VIA THE USE OF EMER CHKLISTS; WHILE I (FO) FLEW THE AIRPLANE AND HANDLED ATC COMS. WHILE THE CAPT WORKED THROUGH THE EMER CHKLIST; ATC CALLED REQUESTING THE NORMAL PERTINENT INFO FOR THE FLT (IE; SOULS ONBOARD; FUEL; AND STATED INTENTIONS). I RESPONDED WITH THE INFO AND REQUESTED AN IMMEDIATE RETURN TO LAND USING THE ILS TO RWY 21L AT DTW. THE CAPT INFORMED OUR COMPANY OF OUR RETURN; DISCUSSED IT WITH THE LEAD FLT ATTENDANT; AND INFORMED THE PAX OF OUR RETURN AS WELL. HE ASKED ME TO HAVE ATC SEND OUT THE EMER EQUIP. I ASKED HIM IF HE WANTED ME TO DECLARE AN EMER AT THAT POINT. HE SAID NO. I STATED TO HIM THAT I THOUGHT WE WERE REQUIRED TO; IN ORDER TO REQUEST THE EMER EQUIP; AS WELL AS IT BEING NECESSARY GIVEN OUR SIT. AGAIN HE SAID NO. AT ABOUT THIS TIME THE FLT ATTENDANT CAME BACK TO THE COCKPIT AND STATED THAT SHE NO LONGER SMELLED THE FUMES. THE CAPT ELECTED TO RETURN TO THE ARPT ANYWAY AS A CONSERVATIVE MEANS TO HANDLE THE PROB. I CONCURRED WITH HIS DECISION. AS THE CAPT CONTINUED TO WORK THROUGH THE CHKLISTS AND PERFORMED HIS DUTIES; I BEGAN TO SET UP AND CONFIGURE FOR THE RETURN TO LAND. SINCE HE WAS BUSY; I ELECTED TO CONFIGURE THE ACFT UTILIZING NORMAL CHKLISTS (READING THEM TO MYSELF) AND PREPARED TO LAND. BY THE TIME HE WAS FINISHED; I WAS ALREADY ESTABLISHED ON THE FINAL APCH COURSE. UTILIZING THE APCH DATA CARDS; I POINTED OUT TO HIM THAT THIS WOULD BE AN OVERWT LNDG. HE NOTICED THAT THE PROC FOR AN OVERWT LNDG REQUIRED THE DECLARATION OF AN EMER; BUT SINCE WE WERE SO CLOSE TO LNDG; HE STILL ELECTED NOT TO DECLARE. THE ACFT LANDED UNEVENTFULLY WITH THE EMER EQUIP STANDING BY. THEY PERFORMED AN EXTERIOR INSPECTION AND FOUND NOTHING TO BE WRONG. WE CLRED THE ACTIVE RWY AND RETURNED TO THE TERMINAL AND DEPLANED NORMALLY. THE FAA REQUIRES THAT ANYTIME FUMES OR SMOKE ARE PRESENT THAT DONNING THE OXYGEN MASKS ARE REQUIRED. SINCE I NEVER SMELLED OR SAW SMOKE OR FUMES; IT NEVER OCCURRED TO ME TO DON THE MASK. ADDITIONALLY; OVERWT LNDGS REQUIRE THE DECLARATION OF AN EMER. I INFORMED THE CAPT THAT I BELIEVED THAT IT WAS NECESSARY TO DECLARE AND HE ELECTED NOT TO. WHAT STARTED OUT TO BE A SEEMINGLY URGENT MATTER PROGRESSED INTO A NON-URGENT MATTER. AS THINGS BECAME LESS URGENT; WE SHOULD HAVE TAKEN THE TIME TO MAKE SURE THAT EVERYTHING ON THE EMER CHKLISTS WERE COMPLIED WITH; INCLUDING THE DONNING OF THE MASKS AND THE DECLARATION OF AN EMER.