Experimental reports continue to suggest that the crystalline solid state may present a unique environment for deuteron-deuteron (d-d) fusion at ambient temperature (cold fusion). The analysis herein shows that newly reported cluster-impact d-d fusion at energies ∼100 eV has much in common with coldfusion and might appropriately be called lukewarm fusion. Both phenomena evidently need a novel theoretical approach for their understanding. A deuteron effective mass approach is proposed as a possible explanation of the reported experimental results.