Got to remember, especially in the absence of cold aisle containment, temperature inside the equipmnet will be notably higher than ambient.
Particularly with networking kit it seems, the first few generations of MPLS core routers were notably bad, we still use a fair few Foundry 8 slot XMRs, which vent hot air out the side (and it's properly hot, 10Gbit line cards seem to run at 60 degrees+ even when the room temp is 20). I don't really know what their engineers were thinking to be honest, it's a nightmare to keep them cool. Lower end Juniper M series boxes had/have the same issue but don't generate so much heat.
That said most new kit has front to back cooling now so it's less of a problem.