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Updated WHO specifications for netting materials and mosquito nets - page 9 / 34

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Technical consultation on specifications and quality control of netting materials and mosquito nets

3. MINIMUM GENERAL SPECIFICATIONS FOR MOSQUITO NETS

3.1 Mesh count

Available evidence for the efficacy of non-treated mosquito nets is mostly based on warp-knitted polyester netting, with a minimum mesh count of 156 (i.e. 156 holes per square inch). Although the holes in netting with this mesh count are usually neither round nor square, and may be somewhat irregular in shape and size, they have a diameter of about 2 mm and provide an effective barrier to most Anopheles mosquitoes. Netting with a mesh count of 196 (i.e. 196 holes per square inch) is commonly used for prevention of leishmaniasis, which is transmitted by sandflies that are smaller than mosquitoes. Netting for LLINs may have larger holes (e.g. 56 holes per square inch), because such netting presents both physical and chemical (repellent and killing) barriers, which are expected to remain effective throughout the normal service life of the net.

In the absence of an international standard method, mesh count is as- sessed visually with a Pick glass or other accurately calibrated rectangular template, or with a ruler. The assessment may be made directly on the material or by scanning/photocopying a measured square of the netting and counting the holes on an enlarged printout. Counting is facilitated by aligning one edge of the Pick glass or template with the holes in one dimension (e.g. lengthwise) and, if possible, aligning the adjacent edge with the holes at right angles (i.e. width wise), so that it is necessary to count only the holes on two sides of the square. Alternatively, complete holes can be counted along a ruler in the two directions of the fabric at a right angle, using a multiplication to arrive at surface area. Irrespective of the method used and the alignment or otherwise of the template, part-holes ½ are counted as complete holes, whereas part holes <½ are not counted. Errors in counting are likely to arise if the area counted is not square or rectangular. In practice most counting methods produce similar, but not necessarily identical, results.

Consensus was reached that, for protection against mosquito entry, the size of holes is more important than the number of holes per unit surface area. However, given the wide range of hole shapes in various fabrics and the absence of a standardized method for measuring the dimensions of holes of irregular shape, the 156 mesh count remains the quality specification for mosquito control for untreated or field-treated netting on condition that the netting has holes of reasonably homogenous size. As already indicated, because of their retention of insecticidal activity against mosquitoes, LLINs may have a different minimum mesh count, which may be product-specific.

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