Unfortunately, standard ITNs must be replaced or re-treated with insecticide after six washes and, therefore, are not seen as a convenient, effective long-term solution to the malaria problem. As a result, the mosquito netting and pesticide industries developed so-called long-lasting insecticidal mosquito nets, which also use pyrethroid insecticides. There are three types of LLINs — polyester netting which has insecticide bound to the external surface of the netting fibre using a resin; polyethylene which has insecticide incorporated into the fibre and polypropylene which has insecticide incorporated into the fibre. All types can be washed at least 20 times, but physical durability will vary. A survey carried out in Tanzania concluded that effective life of polyester nets was 2 to 3 years; with polyethylene LLINs there are data to support over 5 years of life with trials in showing nets which were still effective after 7 years. When calculating the cost of LLINs for large-scale malaria prevention campaigns, the cost should be divided by the number of years of expected life: A more expensive net may be cheaper over time. In addition the logistical costs of replacing nets should be added to the calculation.