On Tuesday the Pollutant Standards Index (PSI) reached "unhealthy" levels, according to Singapore's Maritime Port Authority (MPA).
"It has been reported that the average 24-hour PSI level is between 101 and 200. This PSI level range is considered as unhealthy," it said.
The MPA advised the local maritime community to keep an eye on PSI levels and to monitor their outdoor workers and follow guidelines issued by the Ministry of Manpower (MOM).
