According to the US embassy classification, anything beyond 300 is categorised 'Hazardous' warranting people to take preventive measures like avoiding outdoor exposure. Few students showed up at the Beijing No. 2 Experimental Primary School yesterday as the school sent out text messages giving them the choice of either staying home or coming to school.Car use will be limited during the red alert period as cars are allowed on the roads on alternating days depending on the odd or even numbers of their license plates.