tarnish
Is a chemical reaction caused by silver coming in contact with sulfur compounds, mainly hydrogen sulfide, in the air resulting in silver-sulfid. Relative humidity also affects tarnish rate.
Relative Humidity
For a given concentration of airborne hydrogen sulfide, silver tarnishes faster as the relative humidity increases. Silver is best stored in dry conditions, below 50% relative humidity. Gel silica packets can be used in airtight bags or boxes to effectively reduce humidity and minimize airborne sulfur exposure.
Common tarnishing causing culprits:
Foods: eggs, and foods that contain eggs such as mayonnaise as well as acidic foods like lemon, ketchup, or salad dressings
Water: sulfur-rich well water, seawater, hot tubs, swimming pools
Fossil Fuels: natural gas, coal, oil
Rubber: bands, carpet padding, o-rings, bottle stoppers
Smoke: tobacco, fireplaces, candles
Fabrics: wool, fabrics treated with fire-retardants, and most leathers
Cosmetics: lotions, perfumes
Building materials: certain adhesives, insulating foams, grout
Short exposure to any of this is not a problem, but care should be taken not to leave silver coated with egg, granules of salt, or stored held together by a rubber band.
Eggs are the frequent annoyance for well used silver
The most common sulfur-containing gas is hydrogen sulfide (H2S), which is the one given off by a hard-boiled egg. Understanding How Objects Tarnish by Lyndsie Selwyn provides more details on how silver tarnishes using hardboiled eggs in an experiment.