It seems that some increase in loudness is inevitable when going to higher notes. Is there any widely known recommendation or rule of thumb from the voice experts as to how many decibels can a singer go above his ordinary conversation voice without risking damage to vocal cords ?
As you practice, you will find a point where, to get more volume, you stop feeding air through your vocal chords with the diaphragm, and start using the ribcage and other muscles in the chest to force air through your throat. The feeling of this, and the sound produced, may be described as feeling or sounding "overblown" compared to proper technique, similar to the same thing done to a brass or wind instrument. This border is your "red-line"; just to the "safe" side of it is your natural fortissimo; it's loud, but you could do it all night, or most of it anyway. Using subtle variations of technique to "focus" the tone at this volume level can make a VERY high-volume tone.
Beyond that line, where you start forcing air out of your throat any way you can, is your triple- or quadruple-forte. It's akin to a fighter pilot hitting the afterburner; it gets more speed out of the jet, but actually not as much extra as you might think, and it goes through the jet's fuel at 10 to 20 times the full non-afterburner setting. If you ever sing at this level at all, it should only be used for a few seconds, only once or twice a night, to give a big finish to a piece or for the specific effect of sounding overblown. Overuse of this vocal level, even with proper technique everywhere else, will very quickly fatigue your chords, reduce your range, swell your larynx and generally blow out your voice for the night (and at least a few nights thereafter).
Although singers may go louder on higher notes, as you've noticed, they don't have to - higher notes can be sung more quietly if using solid technique.
I can't give you a figure, but it is possible for singers to create a VERY loud sound without harming themselves, and a good example of this are opera singers, who manipulate various aspects such as larynx position to create greater resonance, one which specifically enhances the frequencies of the pitch they're singing. This is sometimes called the 'singer's formant' and creates a 'louder' sound. As I understand it, the changes they make do not risk their vocal cords.
Singing 'loudly' is not inherently dangerous, it's rather that singing with poor technique (whether that's quietly or loudly) is more likely to damage your vocal cords. So for example if you are pushing the sound from the throat, rather than supporting the sound from the diaphragm or making helpful use of resonance when singing higher, then you are more likely to hurt yourself. Yelling or screaming (as Tom says) can cause damage, but so can a 'normal' speaking voice if it uses excessive tension or pushing.
So in summary, it's not volume, it's technique. As long as you are having good training, I wouldn't worry about whether you're singing 'too loud'!
The three things you surely shouldn't be doing are yelling, screaming or cheering; the increase in loudness when going to higher notes is however just fine. Just never do more than you need to be doing...
If you want an idea of the loudness where it starts to become unsafe, opera singers risk their voice.
They do their best to maintain their voice which allows them to counter the effects of singing so loud.