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The Air Quality Regulations 1997

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Regulations 2(2) and 4

SCHEDULEAIR QUALITY OBJECTIVES

TABLE

SubstanceAir Quality Objective levels
Benzene5 ppb or less, when expressed as a running annual mean
1,3-Butadiene1 ppb or less, when expressed as a running annual mean
Carbon Monoxide10 ppm or less, when expressed as a running 8 hour mean
Lead0.5 micrograms per cubic metre or less, when expressed as an annual mean
Nitrogen Dioxide150 ppb or less, when expressed as an hourly mean, and 21 ppb or less, when expressed as an annual mean
PM1050 micrograms per cubic metre or less, when expressed as the 99th percentile of daily maximum running 24 hour means
Sulphur Dioxide100 ppb or less, when expressed as the 99.9th percentile of 15 minute means

Interpretation

For the purposes of this Schedule:

1.  “ppb” means parts per 1000 million by volume.

2.  “ppm” means parts per million by volume.

3.  “PM10” means the mass fraction of particles which if inhaled would penetrate beyond the larynx. This mass fraction shall be taken to be the thoracic convention, ET, as defined in ISO 7708:1995(E) ‘Air Quality—Particle size fraction definitions for health-related sampling’, published by the International Organisation for Standardisation.

4.—(1) A running annual mean is a mean which is calculated on an hourly basis, yielding one running annual mean per hour. The running annual mean for a particular substance at a particular location for a particular hour is the mean of the hourly levels for that substance at that location for that hour and the preceding 8759 hours.

(2) For the purpose of the calculation of a running annual mean, the hourly level for a particular substance at a particular location is either:

(a)the level at which that substance is recorded as being present in the air at that location during the hour on that basis of a continuous sample of air taken during that hour for at least 30 minutes; or

(b)the mean of the levels recorded at that location on the basis of 2 or more samples of air taken during the hour for an aggregate period of at least 30 minutes.

5.  A running 8 hour mean is a mean which is calculated on an hourly basis, yielding one running 8 hour mean per hour. The running 8 hour mean for a particular substance at a particular location for a particular hour is the mean of the hourly means for that substance at that location for that hour and the preceding 7 hours.

6.—(1) An annual mean is a mean which is calculated on a yearly basis, yielding one annual mean per calendar year. The annual mean for a particular substance at a particular substance at a particular location for a particular calendar year is:

(a)in the case of lead, the mean of the daily levels for that year;

(b)in the case of nitrogen dioxide, the mean of the hourly means for that year.

(2) For the purpose of the calculation of an annual mean, the daily level for a particular substance at a particular location for a particular day is the level at which that substance is recorded as being present in the air at that location during the week in which the day occurs on the basis of a continuous sample of air taken throughout that week (each day in that week therefore being attributed with the same daily level).

(3) For the purpose of paragraph (2) “week” means a complete week beginning on a Monday, except that it also includes any period of less than seven days from the beginning of the calendar year until the first Monday in that year or from the beginning of the last Monday in the calendar year to the end of that year.

7.—(1) A daily maximum running 24 hour mean is a mean which is calculated on a daily basis, yielding one daily maximum running 24 hour mean per day. The daily maximum running 24 hour mean for a particular substance at a particular location for a particular day is the maximum of the 24 running 24 hour means recorded for that substance at that location during that day.

(2) For the purpose of the calculation of a daily maximum running 24 hour mean—

(a)a running 24 hour mean is a mean which is calculated on an hourly basis, yielding one running 24 hour mean per hour, and the runnning 24 hour mean for a particular substance at a particular location for a particular hour is the mean of the hourly means for that substance at that location for that hour and the preceding 23 hours;

(b)“day” means the 24 hour period commencing immediately after midnight, Greenwich mean time.

8.—(1) The nth percentile (that is to say the 99th or the 99.9th percentile) is calculated for a calendar year (yielding one result per calendar year) as follows:

  • Let:

(1) N equal the number of measurements for that calendar year;

(2) k equal—

rounded to the nearest whole number (or rounded up to the nearest whole number where the fractional part of the number is 0.5); and

(3) the N measurements be listed in ascending order:

  • X1; X2; X3; ... ... ... ... Xk; ... ... ... ... XN1; XN

  • Then: the nth percentile for that calendar year is the value of the measurement listed in the kth position; that is Xk.

(2) For the purpose of the calculation of the nth percentile, the measurements for a calendar year are:

(a)in the case of the 99th percentile of daily maximum running 24 hour means, the 365 daily maximum running 24 hour means for the calendar year in question (or 366 in the case of a leap year);

(b)in the case of the 99.9th percentile of 15 minute means, the 35040 15 minute means for the calendar year in question (or 35136 in the case of a leap year).

9.  A 15 minute mean is a mean calculated every 15 minutes. The 15 minute mean for a particular substance at a particular location for a particular 15 minutes is the mean of the levels recorded, at a frequency of not less than once every 10 seconds, for that substance at that location during that 15 minutes.

10.  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, at a frequency of not less than once every 10 seconds, for that substance at that location during that hour.

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