Whether noise will harm your hearing depends on a combination of factors such as volume, exposure time, frequency and individual susceptibility.
A single episode of very loud noise can do damage, but so can not-so-loud noise that extends over a long time.
Decibel readings offer the most convenient way of measuring noise exposure. For context: Ordinary conversation between two people is approximately 60 decibels. At the other end of the spectrum, the pain threshold -- where damage can be done quickly and where you'd be inclined to cover your ears -- generally starts at around 120 decibels, researchers say.
If you're interested in monitoring how much noise you're being exposed to, there are some free or inexpensive noise-measuring apps available both for iPhones (among them Decibel Ultra, NoiSee, dB Meter, SPLnFFT Noise Meter and Noise Hunter) and Android phones (including Sound Meter, Noise Meter and deciBel).
Here are approximate decibel values for some things that are louder than you might think.
Baby rattle: 81
Food blender: 88
Salvation Army holiday bell-ringing: 91
Lawn mower: 95
Leaf blower: 95-105
Hand drill: 98
Football game: Up to 99
High-speed hand dryer: 80-100
Hair dryer: 80-95
Indianapolis 500 car race: 104-109
Beats by Dre headphones (turned up to maximum volume): 115
Referee's whistle: 104-116