Atmospheric escape is a key process controlling the long term evolution of planets. Radiative cooling competes for energy against atmospheric escape in planetary upper atmospheres. In this work, we use a population balance method and a Monte Carlo model to calculate the previously ignored emissions of metals (C, N, O and their ions) and compare them with radiative recombination of H II and Ly-α emission of H I, which are the most efficient cooling mechanisms currently recognized in the upper atmospheres of hot Jupiters. The results show that the emissions of C, N, O and their ions are strong non-linear functions of environmental parameters (temperature, density, etc.) and are likely to be efficient cooling mechanisms in the upper atmospheres of close-in exoplanets.