Tarnish is caused by contact with sulfur compounds, mainly hydrogen sulfide in the air. Relative humidity (RH) also affects the tarnishing rate. Other common culprits are:
Foods (partial list) : eggs, onions, mayonnaise, sour cream, mustard, garlic, poultry, fish, cabbage, bananas, coffee, chocolate, nuts
Water: sulfur-rich well water, seawater, hot tubs, swimming pools
Fossil Fuels: natural gas, coal, oil
Rubber: bands, latex gloves, carpet padding, bicycle tires, o-rings, stoppers
Paint: check with the manufacturer
Light: the sun, light bulbs that off-gas
Smoke: tobacco, exhaust, fireplaces, candles
Fabrics and Leather: most are treated and tanned with sulfur-bearing compounds, wool
Cosmetics: lotions, perfumes
Clays: (e.g. certain modelling clays)
Glues: (protein-based)
Other: sewer gases
Sulfur (elemental sulfur): some adhesives, cement, grout