Methodological issues seem to plague UV-effects research, perhaps because results impinge closely on policy concerns, but also because there are subtle problems associated with the dosimetry involved in accurate quantitative measurements. Considerations associated with the estimation of biological weighting functions, dose response curves, and reciprocity have been discussed above. A continuing critical factor in the methodology of laboratory experiments is that radiation regimes in laboratory experiments cannot easily simulate ambient levels of solar radiation throughout the total spectrum. This is especially true when vertical mixing through an in-water irradiance field is under consideration. As a consequence, while experiments may enhance the UV-B spectral region, the visible portion of the spectrum may be as much as an order of magnitude lower than in nature, possibly limiting the energy necessary for optimum photoreactivation and photorepair [Kaupp Hunter 1981, Damkaer Dey 1983, Worrest 1986 overview]. Further, simulation of in-water irradiance is difficult to match, especially with respect to UV dosimetry, and results are very difficult to compare without the application of BFWs, which is seldom done.
Another methodological concern,
that there may be a UV-B induced toxicity in polyethylene bags which
significantly lowers the rate of 
assimilation,
has recently been raised
[Holm-Hansen Helbling science 1993, Holm-Hansen Lubin Helbling 1993].
This assertion runs counter to previous tests and experience
( [El-Sayed Stephens Bidigare Ondrusek 1990,
Ray Smith Prezelin macintyre 1992,
Prezelin Boucher Smith 1992 daytime],
Karentz and El Sayed, personal communication).
Several additional tests concerning this UV-B toxicity question
[Prezelin Smith Response 1993, Prezelin Boucher Smith 1994,
Prezelin Boucher Schofield 1994, Milot-Roy Vincent 1994, Moeller cullen 1994]
show no toxic effects were evident from the use of polyethylene bags.
Milot-Roy & Vincent, in agreement with Prezelin & Smith, point out that
experimental comparisons using containers of different materials and
optical geometries should be undertaken with caution.
The careful experiments explicitly aimed at evaluating the use of the
operationally more available and easily used polyethylene bags for UV field
work [Prezelin Boucher Schofield 1994] shows conclusively that these
bags can be used for accurate quantitative work and that no toxicity exists.