# The Air Quality (Wales) Regulations 2000, Paragraph 4

Extent: E+W

An hourly mean is a mean calculated every hour. The hourly mean for a particular substance at a particular location for a particular hour is the mean of the levels recorded for that substance at that location:
(a) in the case of benzene, at a frequency of not less than once during the hour; and
(b) in the case of carbon monoxide, nitrogen dioxide and sulphur dioxide, at a frequency of not less than once every 10 seconds during the hour.

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Source: https://www.legislation.gov.uk/wsi/2000/1940/schedule/paragraph/4 (the official version)
Page: https://openlegislation.uk/air-quality-wales-regulations-2000/schedule/paragraph-4
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.