Complex sentence: The PM level in Delhi since the WHO survey have become worse and in Dec-January 2015, an average PM 2.5 level of 226, a level considered very unhealthy, was noted by US embassy monitors in Delhi.
Split into simpler sentences: The PM level in Delhi since the WHO survey have become worse. In Dec-January 2015, an average PM 2.5 level of 226, a level considered very unhealthy, was noted by US embassy monitors in Delhi.