San José has kept the amount of air-borne nitrogen dioxide (NO2) under control in its residential areas but is seeing increases in the industrial sector, a new study on air quality shows.

Released Thursday, the report looked at several air pollutants, including toxins from metals and carbon dioxide between 2004 and 2008, but in their presentation, the report's authors focused mainly on the increase in nitrogen dioxide (NO2) in certain areas.

NO2 is a toxic chemical compound mainly produced by internal combustion engines and manufacturing plants.

The results of the study revealed that San José has kept the amount of air-borne NO2 under control in its residential areas, but has seen increases in industrial sectors.

Heredia, north of San José, has work to do in both areas.

Exact numbers vary greatly because nitrogen dioxide quantities depend heavily on the season, but the tendency has been for San José to lower the amount of in the air by four percent in residential areas.

By contrast, Heredia's residential areas have seen an increase of 18 percent between 2005 and 2008.

The reason for this disparity between San José and Heredia's residential sectors remains in doubt.

Jorge Herrera, Director of the environmental analysis lab for the National University (UNA), who organized the report, said it will take more analysis to find the reasons for the gap between the two cities, but he noted that rapid population growth in Heredia could be an explanation.

According to Infocensos, a University of Costa Rica (UCR) system that gauges populations in Costa Rica's counties, Heredia has boomed from close to 39,000 residents in 1970 to about 130,000 in 2008.

San José's population has also risen but not quite as sharply. During the same time period San José grew from approximately 220,000 people to 330,000.

Herrera also said that San Jose's vehicle restrictions, which prohibit vehicles with certain license plates from entering the city once a week, could also have contributed to the improvement in the city's air.