We report on a joint SUMER, CDS, TRACE study, concentrating on a region which shows prolonged EUV explosive event (EE) activity in the transition region line N v 1238 Å, yet little evidence of such activity in another transition region line O v 629 Å (formed at a similar temperature) which was observed simultaneously. A possible explanation for the lack of major activity in the O v line in several explosive events could be that they originate in the lower chromosphere. This is consistent with the enhancements in the C i 1249 Å line and with the findings of another study which reported time delays between the chromospheric and transition region lines in some EE's using high cadence observations (10 s exposure time) obtained with the SUMER spectrometer in H i Ly 6 (20 000 K) and S vi (200 000 K). Using the generalized collisional-radiative picture, including the population of metastable levels, we derive the density dependent contribution function for both N v 1238 and O v 629 for four values of the electron density; 106 cm-3 representing the low density limit, 109 cm-3 for a typical quiet Sun electron density plus 1011 cm-3 and 1012 cm-3 for an active region. These calculations show that with increasing electron density, both lines shift to slightly lower temperatures. However, the major difference is in the relative increase in the line flux with increasing density. For N v, increasing the density to 1011 cm-3 results in a 60% increase in the line flux, while O v shows a 30% decrease. Increasing the electron density to 1012 cm-3 results in a factor of two decrease in the O v flux, thus making it difficult to detect explosive event activity in this line if the event is formed in the chromosphere. Other explosive events which show simultaneous activity in both lines are probably formed in the transition region. In one such event, activity is observed in both N v and O v, yet nothing in C i. In this event we also observe an increase in the TRACE 173 emission, delayed by ≈40 s compared to the transition region lines.