Sources of positive ions

Household

Air-conditioning and heating units
Carpet and other fibers
Dust, mold, and pet dander
Clothing stored in boxes, etc.
Household appliances such as electric blankets, fluorescent lights, hairdryers and irons, heating pads, television and television accessories like cable boxes, and water machines
Household cleaners and other chemicals
Home office equipment such as computers and printers
Major appliances like clothes dryers and microwaves

Medical

Air pollution
CAT scans
CT scans
Xrays
Other medical scans

Office

Carpet and furniture fibers
Cleaning chemicals
Computers
Fluorescent lights
Photocopiers
Printers
Water machines
Other electrical sources

Outdoors

Air pollution
Exhaust fumes from vehicles
High tension power lines
Lightening before a storm (not the rain!)
Power stations
Radio or TV base stations

Personal items

Car air conditioning and radio
Magnetic bracelets (both +ve and -ve charges)
Smartphones

Diffused oils, incense, and smoke

In addition to burning incense and smoke from cooking fires, the following oils generate positive ions when diffused into the air:

Clove
Cypress
Eucalyptus
Frankincense
Helichrysum
Juniper
Marjoram
Melaleuca (quinquenervia)
Palmarosa
Pine
Ravensara
Rosemary
Thyme
Ylang Ylang