The State Environmental Protection Agency considers readings below 50 to be 'excellent', and up to 100 as 'good'. On days with higher readings, it begins issuing advice on health implications. The highest the index reached during the expo period - which began just before the gates opened on May 1 - was 113, and it went over 100 on only two other occasions. The low readings are believed to be largely due to a moratorium on construction work in the city, clampdowns on heavily polluting trucks and emissions controls on factories in the surrounding region.

The API is the agency's non-linear scale measuring levels of key pollutants including particulate matter, sulphur dioxide and nitrogen dioxide. The day's highest reading is used as its overall figure. The reported measurements for all three pollutants were above 50 for both Monday and Tuesday this week - which occurred on only two days during expo.

The pollution data, from the Shanghai Environmental Monitoring Centre, stands in contrast to the self-congratulatory tone of a statement released by the Shanghai Environmental Protection Bureau, which celebrated air quality as having hit a 'historic high' while the multibillion-yuan fair was running.

The bureau ascribed the improvements to a collaborative agreement with its counterparts in neighbouring Zhejiang and Jiangsu provinces signed in late 2008. It brought stringent controls on power stations, steel mills, chemical plants, construction sites, refineries and food plants within 300 kilometres of the expo site. There was a clampdown on vehicle emissions, as well.

As a result, readings for particulate matter, sulphur dioxide and nitrogen dioxide were all the lowest recorded during the equivalent six-month segment of the year.

Local media quoted bureau officials as saying that after expo, authorities would 'go a step further' in controlling air pollution to ensure that days of 'blue skies and white clouds' were no longer 'a thing of the past'. An employee at the environmental monitoring centre said: 'We feel that air quality levels have shown a noticeable improvement during the expo period.' She directed all further inquiries to the Environmental Protection Bureau, but calls to its spokeswoman's office rang unanswered.

The monitoring centre's figures show Shanghai had API readings below 50 for 90 of the expo's 184 days - 31 more than during the same period last year. Between May and the end of October last year, the index went over 100 eight times.

The improvements in air quality were even more noticeable on days when sulphur dioxide and nitrogen dioxide were particularly low.

Sulphur dioxide levels were lower than 25 for 129 days during the expo, 38 more than during the same period last year. Nitrogen dioxide readings were under 25 for 91 days, 23 more than in the same six months last year.

Blue skies

Shanghai had API readings below 50 on 90 of expo's 184 days, more than the same period last year by this many days: 31