Another type of nanosensor is one that works through a colorimetric basis. Here, the presence of the analyte causes a chemical reaction or morphological alteration for a visible color change to occur. One such application, is that gold nanoparticles can be used for the detection of heavy metals. Many harmful gases can also be detected by a colorimetric change, such as through the commercially available Dräger Tube. These provide an alternative to bulky, lab-scale systems, as these can be miniaturized to be used for point-of-sample devices. For example, many chemicals are regulated by the Environmental Protection Agency and require extensive testing to ensure contaminant levels are within the appropriate limits. Colorimetric nanosensors provide a method for on-site determination of many contaminants.