Air ambulance helicopter pilot reported a near miss during lift off with a crop duster aircraft that flew over the aft part of the helicopter.

Narrative

On Day 0 at approximately XA39hrs myself and med crew were lifting from the company base located ZZZ. Our helipad sits on the north end of the airfield. I made the appropriate pre-lift announcements on the CTAF XXX.X. I cleared my visual line of sight to the north; east and west. The med crew cleared theirs as well. As I was lifting; the flight medic seated left rear loudly announced 'HOLD HOLD HOLD'. I ceased lifting immediately at what I estimate to be 20' AGL; and I saw a shadow cross from west to east. I looked to the east and saw the tail of a white airplane flying low level; I would estimate no more than 100 - 125' at the highest. I tried again on XXX.X to gain the pilots attention; and tried XXX.XY as well. No answer on any frequency.Our typical departure from the helipad is a vertical climb to 100' and then depart to the north; west or east.I understand; and all of the pilots understand that there are crop dusters working the area during this time of year and they do not have ADS-B out or any type of Radio communications.However; with that being said had it not been for the med crew doing their job in an extremely professional manner and making clear and concise announcement to HOLD; this had quite the potential to be a mid air collision. The rear medic advised the plane crossed right over the aft part of our helicopter.I understand that the crop dusters work at low level; however the crossing of ZZZ at such a low level is unacceptable; especially when the pilot of the crop duster plane does not have communications to hear departing and arriving aircraft.I would strongly suggest that all crop duster pilots be required to carry a working hand held portable air to air radio when flying. They are inexpensive and can save lives. I would also suggest that when crossing an airport; glider port etc.; that they maintain at least 500' at mid field. I did speak with the glider port owner; she was unsure exactly which crop dusting company was flying on this day; but believed it could be Company X.I believe that if we can locate the pilot; this can be an educational lesson for him / her and the tragedy that may result for such actions when operating around an airport / glider port or any area that has the potential for departing / arriving aircraft.