UV Degradation

Answers to the same old materials questions ....

My last day managing NIWA’s UV radiation program at Lauder was way back in September 2012. But eight years later I’m still receiving inquiries from manufacturers and suppliers of outdoor products whose lifetime can be compromised by exposure to excessive UV radiation. Here I try to answer their questions for once and for all.

First some background. For some products, UV damage can be minimized by adding blockers. But the manufacturers don’t want to add too much because (a) they cost money, and (b) they can adversely affect other mechanical properties, like brittleness. So, in today’s throw-away consumer economy, manufacturers seek to add just enough to ensure the guarantee period is satisfied. Ideally (in their eyes), it will fail soon thereafter so more of the product can then be sold. But perhaps that’s being a little too cynical.

The reason I receive inquiries from this sector is because of New Zealand’s reputedly high levels of sunburning UV radiation. But that UV radiation is just one of several factors that contribute to our world-record rates of skin cancer. As I’ve explained before, the UV in New Zealand is not particularly high compared with equatorial regions. We do have relatively high UV levels - by about 40 percent - compared with similar latitudes in the northern hemisphere. And it’s about twice as high as in Europe. But the highest UV levels anywhere in the world are nearly double those experienced in New Zealand.

Furthermore, unlike damage to our skin, damage to outdoor materials depends on the annual dose of UV, rather than just the summer peaks. Because the UV we receive at the Earth’s surface depends so strongly on the sun elevation angle, those peak amounts received in the winter at mid-latitudes like New Zealand are only about 10 percent of the summer values. The winter daylight period is also much shorter. As a result, the annual dose at mid-latitudes is much less than in the tropics where noontime solar elevations are always greater than 45 degrees (and day-lengths are more constant), so the daily UV dose remains elevated year-round.

The most relevant work we’ve done on the subject is buried quite deep in NIWA’s Ozone & UV web pages so I’ve summarized it in the plot below, which is based on work we published a few years ago at one of the NIWA UV Workshops that I convened. It uses the highest quality measurements of UV data in the world to show that the further you go from the equator, the lower the annual dose, especially of the more-damaging UV-B component.

Departures from that simple pattern arise from differences in altitude and/or snow or ice cover between sites. The amount of UV received increases by at least 5 percent per kilometre increase in altitude. For example, at Mauna Loa Observatory - altitude 3400m - it is nearly 20 percent more than at sea level. And it can be increased by a further 25 percent or more over snow-covered surfaces, such as in polar regions.

The upper panel shows the measured annual average doses of UVA and UVB irradiance as a function of latitude. The altitudes of each measurement site - in metres above sea level - are also specified. The lower panel shows the ratio of UVB to UVA at each specified site, ranging from 0.03 at the equatorial sites to about half of of that value at mid-latitudes.

The plot suggests that about half the surface area of planet (i.e., all latitudes between 30N and 30S) receives a higher annual doses of UV-B than most of New Zealand, which is approximately represented in the graph by the latitude range from Lauder (45S) to Melbourne (38S). So in terms of the annual dose of UV, New Zealand is below the middle of the pack.

Before UV-sensitive outdoor materials are marketed and deployed they will have typically undergone long-term testing against UV damage in high UV testing sites like Southern California and Florida (latitude ~30N ). Because New Zealand’s annual UV dose is less than those sites, any guarantee against UV damage that applies throughout USA should also apply anywhere in New Zealand.

Next time I receive one of these requests for information, I’ll be able to save a lot of time by pointing the enquirer to this page. Subscribe for more occasional free posts by clicking the button below.