I've been experimenting with harvesting rainwater by use of portable cisterns and exploitation of naturally occurring drainage patterns...well, actually, I have been moving a 45 gallon trash can under places where water runs off the roof. Fills up fast here in the tropic rainy season.

In looking for information about the potability of rainwater, I stumbled upon a publication from academics that indicates that rainwater is actually poison falling from the skies, and needs to be filtered to 5 microns for removal of lead, and 1 micron for bacteria and parasites.
So much for catching raindrops on my tongue.
This level of filtration is well within reach of my msr miniworks and similar devices, but chlorine, iodine and uv filtration seem to be ruled out.

If the rain is as severely polluted as this, why aren't we all dead?

This product http://www.rei.com/product/890899/sawyer-mini-water-filter-package-of-2 filters to 0.1 micron,and I don't believe it is unique. Giving your reference a very quick read, it looks like most of the contaminants comes from contact with roofs

Chemicals such as Lead are dissolved in the water and CANNOT be filtered out.

Distillation is the only way to remove them and that will not remove many of the lighter molecules totally, but perhaps bring levels down to an acceptable level.

Pop along to your local water treatment plant or your water utility and ask them, try and get a tour of the plant and you will see what is necessary.

My understanding of roof runoff is that it varies based on where you live and your roof.

Some folks let the first few minutes of rain go into the ground and then collect the runoff. Some (and I don't advise this without lab testing at least once) do not treat the water collected this way before drinking it. Others treat with filters or chemicals, and I'm sure there are situations where roof runoff cannot be treated economically.

I would suggest either using roof runoff for non-potable purposes or springing for a lab test of both the first water that comes off the roof and water that comes off after five minutes or so of a good rain, to see what you're up against.

The CDC thinks that lead can be filtered out of water.

From the CDC drinking water article


You may also wish to consider water treatment methods such as reverse osmosis, distillation, and carbon filters specially designed to remove lead. Typically these methods are used to treat water at only one faucet. Contact your local health department for recommended procedures. If you want to know more about these filters, please contact NSF InternationalExternal Web Site Icon, an organization for public health and safety through standards development, product certification, education, and risk management. Remember to have your well water tested regularly, at least once a year, to make sure the problem is controlled.

The NC study in the OP did not mention a control harvest to collect rainwater without the roofing product contamination. Raindrops form around high altitude particulates, some of which are contributions from coal-fired industrial plants, jet contrails, volcanic eruptions and angel feathers. Without intervening roof pollution, acid rain has produced a lot of crystal clear lakes in the Adirondacks: crystal clear because they are lifeless due to elevated pH. Reinforces the recommendation for water analysis. I believe government subsidies are available for rainwater cistern installation in the Florida Keys, to reduce demand on the pipe from the mainland-maybe this is a partial answer to the increasing scarcity of water in the West.

Biosand filters and roof water.

Lots to consider. Each roofing material requires thought.

http://slowsandfilter.org/ssf_faq.html#roof_water

Nursemike, you are confusing two styles of filtering.

Physical filtering to remove particulates, which will not work, and exposing the water to absorbents in a bed which, with the correct absorbent, activated carbon, ion exchange resin, etc. will work.

The OPs link statement that filtering to 5 microns will remove dissolved lead is just plain wrong. Very easy to test. Try adding some food colouring dye to the water and see if a particulate filter removes the dissolved material.

Presuming that we are talking about the collecting and drinking of rainwater in an emergency, rather than under normal conditions, then I would not worry overmuch about lead in the water.

Lead is a long term chronic poison and unless present at most improbable levels is of little short term concern.

Presuming that the roof is not made of lead, then the only source of lead in the water is industrial air pollution.

Whilst times are normal, I would drink city water, or deep well water, or bottled drinking water.

In a short term but serious emergency I would drink roof water after boiling or chlorinating to kill germs, and not worry about lead.

In a long term TEOTWAKI situation then industrial activity and the pollution that it produces would cease within a few months.

When collecting roof water, it is desirable to discard the first flow of water from the roof as this will be contaminated with bird droppings, dust, and leaves etc.
Automatic devices to do this can be improvised.
If water be very scarce, then this first flush of water can be put on the garden or used to flush a toilet.

Rather than investing in some system to purify roof water from lead or whatever may be up there, why not collect it from something other than your roof? Like use a couple of big tarps to collect it? This is assuming you're not using rain collection as your primary source of water for long term.