An effort to collect dust from surfaces inside 100 Porter Ranch homes and two schools has been completed by the Los Angeles County Department of Public Health, and now officials will focus on indoor air as part of a second phase to test for chemicals left over from a massive natural gas leak.

Phase two began Tuesday and will consist of eight days of air monitoring. Monitors will be set up in 16 homes per day, said Angelo Bellomo, deputy director for health protection at the Los Angeles County Health Department.

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