There are two monitors set up in the Park under the Williamsburg Bridge while painting and other work is being done. There is no signage or other indicators, but we have been told it's Lead that is being monitored. When details are confirmed about the threshold or how the public is alerted, we'll post it. These are gas-powered and small generators are adding their own tinge to the air here - this one is on the north side of the bridge.
In 2020, LES Breathe tested the soil and remediation was recommended.
Here is the EPA on Lead monitoring. This article explains the concept of (µg/m3) in the second paragraph, and here is an EPA fact sheet, from 2016 for lead exposure.
