It has been - e.g., http://www.apa.org/monitor/201.... Because they're caused by ultrafine particulates (something which is harder to track relative to the larger PM10 and PM2.5), it takes long term exposure and accumulation to be better detectable. Also UFPM moves different than PM10/2.5 relative to wind, and is emitted in inconsistent ratios (e.g., from combustion across natural gas, coal, petroleum, etc.). For example, the two main sources of UFPM from driving an internal combustion vehicle are due to combustion (out of the tailpipe), and the metals released from frictional breaking. Since EVs and hybrids tend to rely on regenerative breaking and use less/no gasoline, both sources are addressed.

This research and its scary implications has been going on for quite some time, and ties to ultrafine particulates that are small enough to pass through the blood-brain barrier (e.g., Calderon-Garcuidenas et al, 2008 - , and APA Monitor on Psychology 2012.