Gas Fume Inhalation - harmful? how to minimize risk?

Sam,

Landscaping engines by and large don't have regulated emissions.
Internal combustion engines emit mechanical energy, sound, carbon dioxide, water & heat.

The additional bad stuff can include unburnt hydrocarbons, carbon monoxide, nitrogen oxides, particulates, etc.

Keep engines serviced and well tuned to minimize pollutants.
See if the equipment mfg. can supply an accessory 3-way catalytic converter & heat guard.
Wear a dust mask with activated charcoal & eye/hearing protection & sun block.

You're wise for considering this. By the time you feel problems many times much damage is already done.

Also contact your local osha dept. for additional risk management info.