Is health and safety a common sense? Do you agree?

H&S should be the common sense! However, it can be quite complicated matter due to legal implications or, as the colleague mentioned, technical aspects of a particular industry. Some do require a very good technical knowledge about a process in question. 

I can understand your statement as a common sense, perhaps for every day people doing everyday activities could be true. 

Yes it is. However crucial it is, it's all down to common sense. It is like how life's do taught us in religious way. You do bad things you'll get sin, you do good things you'll get blessing. As how it applies on Health and Safety too. You comply with the good work practices you'll be safe and create a profitable margin. You violate it, you have to face the consequence of the risk. It's all down to the individuals on their level of common sense in perceiving Health and Safety. What I mean is the importance.

Yes, mostly it falls in common sense domain but it has links with industrial and expert understanding too

absolutely yes but that common sense is not very common in this region.  

hence the need for it to be taught

on the other hand, even if it is common, there are things which are not common sense and quite technical, hence the need for specialised training and certifications

A good example of this is driving – how many times do we see someone doing something stupid or dangerous while driving; yet these drivers have gone through the same driving test as everyone else.  They have the piece of paper to say that they are trained and know what they are doing, but it is only with several years of experience that you could say that they might be competent drivers.  Would you allow an employee to jump into an expensive work vehicle or machine without being confident in their driving ability? Health and safety at work should be the same. Until you are confident in the competency and skills of any employee or contractor (documented, of course), then they need policies and procedures in place to provide clear direction and they also need to be monitored or supervised.