More than 80 percent of test samples of drinking water dispensers in office buildings were contaminated with excessive bacteria, according to Shanghai's consumer rights protection department.

The department randomly sampled 30 drinking water dispensers to test the amount of bacteria. The result showed that only five machines, or about 16.7 percent, met the standard, the Shanghai Consumer Rights Protection Commission said on Thursday.

The 25 substandard machines were found to contain excessive levels of CFU (colony-forming units). Of those, 20 machines had more than 300 CFU per milliliter, far exceeding the national limit of 20 per milliliter.

The consumer rights protection commission warned city residents to be more aware of the cleanliness of drinking water dispensers and to clean them regularly.