The fire started in the early morning hours in a stockpile of recyclable metal parts. Several people reported first hearing an explosion before the fire.
People all the way in San Jose reported a chemical-like smell.
"We thought it was one of our copiers acting up, so I actually had to turn off the machine just to make sure," Selina Tam said.
Air quality, already affected by the prolonged dry and stagnant weather pattern reached unhealthy levels. Playgrounds normally packed with parents and young children were nearly empty and some schools kept students inside.
"We made sure our kids exercise is limited and that they're not over exerting their lungs; we want to keep them safe," Horace Mann Principal Zetha Nobles said.
Wednesday will be another Winter Spare the Air day, the 11th consecutive alert called by the Bay Area Air Quality Management District.
This is the second time in five weeks a fire has burned at the plant; this fire is smaller than the previous blaze. The previous fire forced San Mateo, Alameda and Santa Clara counties to issue a health advisory because of all the stubborn smoke. It took crews 19 hours to put out the fire at that time. Investigators are looking into the cause.