The Environmental Protection Administration of Taiwan has provided air quality service by reporting the pollutant standard index (PSI) since 1997. This standard, developed by the USA Environmental Protection Authority, compares concentrations of the five main common pollutants (PM10, O3, SO2, CO, NO2). For each pollutant, a sub-index was calculated from a segmented linear function that transforms ambient concentrations onto a scale extending from 0 to 500. The standard index is based on the highest sub-index. The main disadvantage of the PSI is that it only identifies the levels of one pollutant at a time. Hence it cannot show whether more than one pollutant exceeds the daily standard level. A region may be regarded as polluted when the PSI scale reaches 100; however, this transformation can be misleading. For example, when the standard level for PM10 concentration reaches 125μg/m3, it converts to a less significant value of 88 on the PSI scale. Confusion can also arise because the standard pollution concentration level varies among different countries. This paper discusses a more effective way of determining a suitable concentration level of pollutants in Taiwan. Combining the original PSI with an entropy function, we can develop a revised air quality index (RAQI). The revised version can rectify the current deficiencies of the PSI. It considers the association of the five pollutants, and has the comparative index function. According to tentative results, RAQI should be representative, supplying the public with a better indicator of air quality.