Density Altitude is Pressure Altitude corrected for non-standard temperature. As temperature and altitude increase, air density decreases.
On a hot and humid day, the aircraft will accelerate more slowly, will need to move faster to attain the same lift and will climb more slowly. The less dense the air, the less lift, the more lackluster the climb, and the longer the distance needed for takeoff and landing. Fewer air molecules in a given volume of air also result in reduced propeller efficiency and therefore reduced net thrust. All of these factors can lead to an accident if the poor performance has not been anticipated.
Attached - EASA Sunny Swift: Flight Instructor - Issue 13. This Theory is valid also for helicopter!!
Robinson Helicopter Company Safety Notice 30 - "Loose Objects Can Be Fatal"
Analysis of helicopter occurrence data has identified a number of accidents and incidents where loose items in the cabin have exited the helicopter and contacted the tail rotor. In some cases this has resulted in a complete loss of control of the helicopter.
Pilots and crews must ensure that all items in the cabin are securely stowed before take-off, even if operating with all doors fitted.
Passengers must be briefed on the dangers of loose items in and around the vicinity of helicopters.
Watch RHC Safety Notice SN-30 Loose Objects Can Be Fatal.
Robinson Helicopter Company Safety Notice 44 "Carrying Passengers"
Carrying passenger is an additional responsibility for the pilot in command. Passenger have placed their trust entirely in the hands of the pilot and
should be advised of risks associated with the flight. Risks include pilot experience level, aircraft capability, and operational considerations such as flight over water or night flight.
Carrying a passenger in and of itself increases risk because passengers add workload and distractions.