What is the fine-grained topic of the following text: Most studies investigate the presence of inverted U-shaped environmental Kuznets curve (EKC) using pollutants such as carbon dioxide (CO2), the use of point pollutants such as suspended particulate matter (SPM) is not so popular in literature. Only return the topic.
Investigation of Environmental Kuznets Curve (EKC) using Suspended Particulate Matter (SPM)