Is HIA a Science?

About six months ago, I re-read the Merseyside Guidelines, the leading guidance on HIA in the UK. It's the guidance that every practioner in the UK, and I'm one of them, has read or says they've read. Okay, so first time round I read them quite quickly and I liked what I read. What I hadn't done three years ago, when I was a novice HIA practitioner still learning the ropes, was to read them critically. That's what I did second time around. Imagine my shock, as a signed up champion of science, when on re-reading them I found, at the back, this definition of HIA:

"Is health impact assessment a science?
It is important to emphasise that HIA is not strictly a science.
Having said this, it most certainly draws on a scientific knowledge base.
Scientific evidence on health impacts of specific determinants forms the backbone of this creative, interdisciplinary form of enquiry.
But each HIA is uniquely located in time, space and local conditions though its evidence base can be evaluated, and the rigour with which procedures and methods were implemented can (and should) be assessed.
Uncertainties encountered during the undertaking of HIAs will frequently dictate the need to make assumptions, which may result in challenges to the HIA’s validity: such assumptions are acceptable as long as they are stated explicitly, so that the reader is free to agree or disagree. ”

I agree with every part of this quote except the phrase "HIA is not strictly a science" because for me it describes perfectly what it means to apply scientific knowledge to the real world outside the lab.

For me, both HIA and science in all its myriad forms are "interdisciplinary forms of enquiry"; to misquote John Donne 'No science is an island entire unto itself every science is a piece of the continent, a part of the main'.

HIA for me is:

1. a systematic endeavour;
2. it uses knowledge and methods from other scientific disciplines e.g. epidemiology, sociology, toxicology and biology;
3. it aims for rigour,
4. it actively avoids bias and confounding;
5. the findings of one HIA have some applicability to communities of a similar social and cultural mix as that studied by the HIA; and
6. its results are subject to revision when new evidence or information comes to light that contradicts and refutes previous knowledge.

For me, this quote illustrates a misreading of science as about numbers and quantitative findings and about universal laws that transcend time and space. That works for the physical world (most of the time) but doesn't work so well for our social and cultural worlds where general laws and theories need to take account of local context. In fact even the universal laws of physics have to take account of local context. Science is about systematic enquiry where the resulting knowledge is always provional and subject to revision.

What's even more interesting is that environmental impact assessment practitioners don't have any qualms about calling EIA a science. Neither do economists or sociologists.

But public health professionals and HIA practitioners, in my experience, seem to have a problem with saying that HIA is a public health science? Or maybe I've got it wrong?