Hi Thunderchild
It would be helpful if you would define what you mean by "30"!
However, in simple terms everything is set out in the Approved Code of Practice and guidance that supports the Control of Lead at Work Regulations 2002 - L132.
Three circumstances where you need medical surveillance:
1. Assessment says that exposure is "significant", i.e. half of the occupational exposure limit which for lead other than lead alkyl is 0.15mg per metre cubed - with assessment made on a day that exposure is likely to be at its maximum in terms of an 8 hour time weighted average.
2. Someone's blood lead or lead in urine hits a number that varies depending on the vulnerability of the person - women of reproductive capacity, young persons ie 16 or 17 and everyone else.
3. A "relevant doctor" directs surveillance.
Nurses don't come into the equation except as delegated by a "relevant doctor".
Once you need that "relevant doctor" probably sensible to be guided by what they say until such time as you can justifiably dispense with their services - which in pragmatic terms means that
(a) you don't have anyone with elevated blood in lead or urine in lead levels
(b) you are confident that exposure is still not "significant" or has been reduced to below such level and can be expected to remain so.
I would note that I think that probably only a fraction of those who should be under medical surveillance under these Regulations actually are. HSE publishes statistics for those subject to surveillance which in some sectors are frighteningly small.
Research in California in the 1980s concluded that over half of demolition worker and those employed in scrap yards had substantially elevated lead in blood levels and I don't think that the UK controls this risk much better than the Californians.
When they were refurbishing the Heilanman's Umbrella at Glasgow Central Station the HSE took a very close interest from the start of the project - so you would expect it to be a paragon of virtue! - yet within weeks of paint removal operations starting, people were getting "suspended".
So, if on a specimen project things were going badly you can reasonably expect more problems in scenarios which are less under the microscope.