The search for a 21-year-old B.C. woman with special needs has been expanded to include more personnel and aircraft as two full days of searching has yielded no result.

Joy Zhang, who police say has the mental capacity of a 10-year-old, has been missing since Thursday afternoon, when she walked off with a friend during a school trip to Sasamat Lake in Port Moody.

Coquitlam Search and Rescue commander Dwight Yochim said on Saturday that 18 searchers have combed through all the park's trails since Thursday. While Zhang's friend was found about a kilometre north of Sasamat Lake on Thursday night, Zhang is still nowhere to be seen.

"The most urgent concern is obviously she has no food or water, and not knowing exactly where she is or what she's up to, we're not sure if she's getting the nourishment she needs," Yochim said.

As Zhang's friend also has the mental capacity of a child, she was not able to tell searchers what exactly had happened to Zhang.

But Yochim says it appears Zhang may have injured her ankle and may have tried to walk between the trails in order to head straight back to the lake.

The search crews, which now include 32 volunteers as well as police officers and the Coast Guard, are looking in areas between trails.

Yochim said they have not given up hope, given that the weather has been warm enough that Zhang should be able to survive the nights.

He said there is also no indication that Zhang may have hopped on a bus, or was met with foul play.

He said he hopes Zhang is just not responding because she is wary of strangers.

Zhang's parents, while very distraught, helped with the search effort on Saturday morning by calling out her name, he said.