Tingling in the left arm is extremely unlikely to be cancer. In most cases, numbness in the left arm is caused by radiculopathy or spondylosis. After the nerve root or spinal cord is compressed, it will produce radiating pain and soreness in the innervated area, as well as numbness. It is also possible that a prolonged posture or prolonged repetition of a certain action leads to local muscle spasm, congestion and edema, and the formation of aseptic inflammation, which can also lead to numbness of the arm. There are also some reasons that the arm is compressed when sleeping, which can also lead to numbness.