A senior environmental officials said on Tuesday that the reason air quality assessments by the Shanghai authority and the United States Consulate General in Shanghai are different is because the evaluation criteria they use are not the same.

The consulate began measuring and publishing the concentration of PM2.5 - particulate matter 2.5 microns or less in diameter — on Monday.

The measured concentration of PM2.5 was 152 at 12 noon on Tuesday, according to the US consulate, compared with 122 at Pudong monitoring site and 155 at Putuo, both of which are measured by Shanghai Environmental Monitoring Center.

However, the US consulate gave the air quality appraisal as "very unhealthy" based on the calculated air quality index, 202, while the assessment from the Shanghai center is "favorable".

"The divergence lies in the different basis for evaluation," said Luo Hailin, a deputy chief engineer at Shanghai Environmental Protection Bureau.

"Our evaluation is based on the air pollution index, which looks at sulfur dioxide, PM10 and oxynitride, while their assessment is based on AQI, which focuses on PM2.5," he said.

The US consulate said the readings indicate the air quality in the area surrounding its downtown offices and are published to make a health data available to the US community in the municipality.

"The monitor is an unofficial resource for the health of the consulate community. Citywide analyses cannot be done on data from a single monitor," read a notice on its official micro blog on Monday.