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Latex films are barriers to viruses.

The article by Roland (pages 15-18) on the barrier performance of latex rubber raises some valid points. The central thesis of the article, however, that thin latex films are inherently pervious to particles the size of the AIDS virus, is not supported by the evidence available.

Condom usage

At the outset it must be accepted that the use of condoms is not 100% effective in preventing transmission of AIDS, just as it is not 100% effective in preventing conception. We contend that the main reason for this inefficiency is the relatively high incidence of misuse, and not any inherent defect in the product. Misuse, in this context, covers a range of eventualities including damage caused by fingernails, jewelry or unacceptable lubricants, failure to use the condom before first genital contact, and slippage during withdrawal of the penis. These are known to be important factors affecting condom efficiency, and explain why the pregnancy rate among condom using couples far exceeds the incidence of holes in condoms. Data of an epidemiological nature showing the apparent spread of AIDS among condom users should be considered with this in mind. There is certainly a need for better education in the correct use of condoms. Roland points to the relatively small amount spent by the condom industry on advertising. This is surely not because of a lack of desire to promote the product but rather the reluctance of advertising regulatory authorities to permit such advertising.

Do pores exist in latex films?

During the manufacture of condoms and other dipped latex products, the possibility of holes occurring in the latex film clearly exists. Condom manufacturers try to minimize the occurrence of holes by employing good manufacturing practices and by electrically testing every condom. Leaving aside this quality control question, it is important to know whether latex films of condom thickness which are made properly, contain pores of a size which could allow passage of the AIDS virus.

Evidence in favor of the pore hypothesis

Roland refers to microscopy work carried out in these laboratories which shows the presence of a capillary structure in latex films, Such a structure may well exist in air dried latex films, but it is equally well known that the porous structure effectively disappears after leaching in water part of the normal condom production process. The interstitial pathways referred to, which allow some absorption of water by latex films, are considered to have dimensions measured in nanometers rather than microns. The one claim of direct observation of a sufficiently large channel in a latex product has not been repeated and has been attributed to an artifact by scientists familiar with microscopic examination of latex rubber films (ref. 1 ).

The other evidence presented in support of the pore hypothesis falls into two categories. The first can be described as anecdotal or epidemiological which, as discussed above, should be treated with caution. The other evidence is based on physical properties. The work referred to by Roland on physical properties alludes to the presence of flaws in molded rubber specimens. It is important to note that these flaws are unrelated to the interstitial pathways or particulate structure mentioned above. Indeed, all of the studies on this subject have been carried out on dry rubber in which a particulate structure does not exist.

The physical nature of the flaws in dry rubber is not known, but they have not been envisaged as holes or voids. Although the flaws have not been observed directly, they affect physical properties as if they were discontinuities or razor cuts on the order of 10 microns long. A channel large enough for the AIDS virus to pass through a condom would need to be at least about 60 microns long with a measurable width. Thus, even if the flaws postulated from physical properties do exist as pores, they are much smaller than would realistically be required.

Evidence contradicting the pore hypothesis

If pores were present in condoms, they would be expected to have a detrimental effect on tensile strength. It is worth remembering that condoms, like many other latex products, commonly have elongations at break in excess of 800%. The thickness of a condom is reduced to 6-10 microns under such an extension and any pore size is multiplied accordingly. The fact is that condoms, even under such high elongations, generally exhibit tensile strengths close to 30 MPa which is at least as good as, and usually better than, thicker products made from the same polymer. It is difficult to see how such strength could be obtained if pores of 0.5 micron diameter were present in the unstretched films.

The ideal way to settle a dispute about the possible existence of pores in condoms is by direct experimentation with the HIV virus. A search of the scientific literature reveals that such experiments have been carried out a number of times. In one study by Van der Perre et al (ref. 2), six commercial brands of condoms were tested in vitro with HIV at a concentration of [10.sup.6] particles per millilitre. The virus-containing medium was placed inside the condoms and subjected to vigorous mechanical movements and changes in hydrostatic pressure. In no case could the virus be detected in the external culture medium. In a similar study by Conant et al (ref. 3), it was found that two other retroviruses completely failed to pass through three types of latex condoms. Minuk et al (ref. 4) tested five brands of latex condoms and one sheep intestinal membrane condom for permeability to radiolabelled Hepatitis B antigen (50% of the size of the Hepatitis B virus). A mechanical vibrator was used to simulate service. All five latex condoms proved to be totally impervious to the antigen, but it did pass through the intestinal condom. The three independent reports described above provide convincing proof that intact condoms provide a good barrier to HIV and to other micro-organisms. Further studies of a similar nature, with similar results, have also been reported (refs. 5 and 6).

Testing of condoms for holes

While accepting that existing methods are adequate for detecting holes large enough for the human sperm to pass through, Roland justifiably points out deficiencies of both the water leak test and the electrical conductivity test in detecting holes of the order of 10 microns and less. The 300 ml water test has in fact been replaced by a one litre test which is slightly more severe. Nevertheless, the sensitivity of the test could be improved by the simple measure of introducing a small amount of an appropriate surfactant to the testing water and standards authorities should consider introducing this modification.

Notwithstanding the deficiencies of the test methods, it is difficult to envisage how a hole of ten microns or less in diameter could occur in a condom. Condoms are made essentially of two rubber layers, each of which is 30-40 microns thick. Since wee are convinced that holes are only caused by foreign particles or air bubbles and are not an intrinsic feature of latex films, a particle of about 10 microns in size could at worst make a hole in one layer of the condom. The possibility of a hole being made through both rubber layers, or of a hole in each layer being perfectly aligned seems extremely remote.

Summary and conclusions

Natural latex condoms, if made properly, provide an effective barrier to HIV. This is not only an expectation based on our knowledge of latex films, but has been demonstrated experimentally by a number of in vitro studies. Failure of condoms due to extraneous circumstances or incorrect use is the main reason for their less than complete efficiency in preventing transmission of HIV. Other latex products, e.g. gloves, have essentially the same composition and structure as condoms but are thicker and are therefore even less likely to permit the passage of viruses.

While it is accepted that current test methods are incapable of detecting micron and sub-micron sized holes, the possibility of such holes occurring in a double-dipped film of condom thickness seems very remote.

We believe that it is misleading and possibly dangerous to those involved in AIDS to suggest that condoms are intrinsically pervious to HIV.

References

1. T.D. Pendle and A.J. Cobbold, Nature, 336, 317 (1988).

2. P. Van de Perre, D. Jacobs and S. Sprecher-Goldberger, AIDS, 1, 49 (1987).

3. M. Conant, D. Hardy, J. Sernatinger, D. Spicer and J.A. Levy, J. Amer. Med. Assoc., 255, 1706 (1986).

4. G.Y. Minuk, S.E. Bohme and T.J. Bowen, Ann. Internal Medicine, 104, 584 (1986).

5. F.N. Judson, J.M. Ehret, G.F. Bodin, M.J. Levin and C.A.M. Rietmijer, Sexually Transmitted Diseases, 16, 51 (1989).

6. S.M. Sasny, N.M. Gantz and J.L. Sullivan, Third Int. Conf. on AIDS 1987, Washington, D.C.
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Article Details
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Author:Pendle, T.D.
Publication:Rubber World
Date:Jun 1, 1993
Words:1475
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