LOWELL — An electrician working on the Thorndike Exchange construction project Wednesday morning suffered significant injuries after falling about 50 feet through an opening in the roof, Lowell Fire Chief Jeff Winward said.

The unidentified man was transported via medical helicopter to a Boston hospital. His injuries did not appear to be life-threatening, Winward said.

The electrician was on the roof at the five-story building when the incident occurred. The worker apparently stepped on to what he thought was another secure area of the roof, but instead stepped on to a covering that hid the opening.

Because the opening was narrow, he was able to slow himself down and prevent further injury, Winward said.

The former mill and retail outlet is being developed into a high-end residential and commercial project by Lawrence-based developer Sal Lupoli, who is a Chelmsford resident.

Follow Rick Sobey on Twitter @rsobeyLSun.