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"For a long time, most ecologists didn't treat urban areas as 'natural' environments, because humans lived in them. ... If we don't believe humans are part of the natural world, we're in trouble."

Cities often have lower pollen counts than rural areas, but pollution in urban areas can make symptoms worse.

Most come from low-income urban areas or isolated rural territories where the Colombian state has a limited presence.

Until this week, California’s agricultural sector had largely escaped the large-scale raids that the Department of Homeland Security has deployed in urban areas, most recently in Los Angeles and Orange counties.

The expansion into rural communities follows days of coordinated raids in urban areas of Los Angeles County, where authorities have targeted home improvement stores, restaurants and garment manufacturers.