Ethyl Corporation originally developed methylcyclopentadienyl manganese tricarbonyl ('Ethyl' MMT) as an antiknock additive for unleaded gasoline. 'Ethyl' MMT is a very effective antiknock compound and is economically attractive compared with achieving antiknock quality by extended refinery processing. This extended processing uses more crude oil and leads to more aromatic hydrocarbons in the finished gasoline. These aromatics are precursors to aldehydes, phenols and polynuclear aromatic hydrocarbons (PNA's) and tend to increase smog and cause eye irritation. Ethyl believes that the use of MMT (up to 0.125 g Mn/gal) in all gasoline used in the United States today would pose no significant health hazard to the general public. The purpose of this document is to outline the reasons for that belief, especially as they relate to MMT itself.