2.3 Results
Table 1 summarises the results for dioxin samples collected at each site over 18 months. They indicate that, on a regional basis, ambient dioxin levels are very low.
The results are reported as femtograms per cubic metre (fg/m3): one femtogram is 1 x 10-15 grams or 0.000000000000001 g. To put this extremely small measure in perspective, 1 fg is to a gram as 1 milligram is to one million tonnes.
Table 1: Dioxin concentrations at each site
| Site | No. (and length) of samples | Total concentrations (fg/m3) |
| Minimum | Average | Maximum |
| Siding Spring | 15 (24-day) | 3 | 17 | 43 |
| Warrawong | 33 (12-day) | 40 | 160 | 320 |
| Westmead | 34 (12-day) | 83 | 480 | 1500 |
At both Warrawong and Westmead, higher concentrations were observed in winter than in summer. This is probably due to a number of factors, including poor dispersion because of the effects of inversion layers and possibly increased inputs from domestic heating.
Dioxins usually occur as a complex mixture of congeners. To enable the relative toxicity of such a mixture of compounds to be expressed as a single number, the concept of toxic equivalents (TEQ) has been developed. In this report, toxic equivalents are based on the international toxic equivalents scheme (I-TEQ).
The toxicity of each congener is given a toxic equivalent factor (TEF) rating. The most toxic congener-2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD)-is rated as 1. Other congeners are rated between 0 and 1, depending on their relative toxicity.
The total toxicity of a sample is calculated in two steps. Firstly, the concentration of each congener in the sample is multiplied by its TEF. Next, the total toxicity is calculated by adding together the contributing toxicity of each congener. This standardises the measurement so it can be used to compare results with other sites and international levels and goals.
Table 2 shows the results obtained from the three monitored areas, expressed as I-TEQs.
Table 2: Dioxins-toxic equivalents at each site
| Site | International toxic equivalents - total (fg/m3) |
| Minimum | Average | Maximum |
| Siding Spring | 0.2 | 0.64 | 2.4 |
| Warrawong | 3 | 10 | 20 |
| Westmead | 2.3 | 14 | 53 |
Page last updated: 26 February 2011
