An exhaust pipe (100) for a fuel burning engine including a hollow body, the body having an internal surface (120), an external surface (110), a first open end (130), a second open end (140) and a longitudinal axis, wherein the internal surface (120) is shaped to form standing cyclic wave geometry having at least 2 cycles (c). When gas containing inhalable particles (22) enters the exhaust pipe (100) through the first open end (130) and flows out of the exhaust pipe (100) through the second open end (140), a substantial amount of inhalable particles (22) are grouped to form filterable particles (182).