Each Local Authority in Kent operates a number of diffusion tube sites to monitor nitrogen dioxide. While they offer a relatively cheap and easily operated method of assessing pollution levels, they are not as accurate as continuous monitors. Consequently, results shown elsewhere on this web site are only taken from continuous monitors. Different analysis laboratories may produce different results, so care must be taken when comparing pollution levels between districts that use different laboratories. Further information on the use and limitations of diffusion tubes can be found in Chapter 1 of the network's 2003 annual report.