More than 850 runners joined the 20th anniversary of the Chinatown 5K on Saturday, undeterred by days of hazardous air quality that caused cancellations of many outdoor events in Chicago. On Thursday, smoke from Canadian wildfires cloaked the Chicago area in a thick, uncanny haze. Government agencies issued health warnings and urged people to stay indoors. Although conditions improved by Friday afternoon, some parks and venues remained closed. R…