FRANKFURT - Global demand for air freight returned to slight growth in August but the challenges that led to declines in June and July still pose a risk for the coming months, a leading trade body said on Wednesday. Air freight volumes edged up 0.2 percent compared with the previous year, against a 0.6 percent decline in July, the International Air Transport Association said in its monthly air freight report. IATA said rising volumes in the Middle East, Africa and Europe helped offset a slide in the Americas and Asia-Pacific. “Some of the key reasons for the earlier weakness - for example, downgraded growth expectations in emerging Asia, and the rebalancing of the Chinese economy toward domestic consumption - are still there,” IATA Director General Tony Tyler said in a statement. “Even though world trade volumes have slightly picked up, the industry will have to work hard to match the strong finish to 2014,” he said.