To detect blood the smell first has to reach the shark.
From The Naked Scientist:
Water molecules in general are carried to the shark by water currents.
If there are no water currents then it is molecular diffusion, the
random movement of molecules that disperses the odour away from the
In general the travel time of odour depends entirely on the local
Near the water surface water velocities in the ocean
can range between a few centimetres per second on a very calm day and
several metres per second in a strong current.
From the ReefQuest Center for Shark Research:
A shark's lateral line system enables it to detect subtle water
movements. Therefore, when a shark's acute olfactory system detects an
attractive chemical, all it needs to do is turn into the current.
Sooner or later, this will bring a shark to the source of the odor.
From PBS - Jean-Michel Costeau Ocean Adventures:
The notion that the mighty great white can smell blood from a great
distance has been central to modern shark mythology.
They can detect some scents at concentrations as low as
1 part per 25 million, which translates to about a third of a mile
away in the open ocean.
From an Interview with shark expert Samuel Gruber:
It comes down to the concentration of the stimulating chemical at the
nose's receptor cell that determines if an animal will detect a smell.
That level is parts per million in sharks.
From the American Museum of Natural History:
The lemon shark can detect tuna oil at one part per 25 million--that's
equivalent to about 10 drops in an average-sized home swimming pool.
Other types of sharks can detect their prey at one part per 10
billion; that's one drop in an Olympic-sized swimming pool!
sharks can detect these low concentrations of chemicals at prodigious
distances--up to several hundred meters (the length of several
football fields)—depending on a number of factors, particularly the
speed and direction of the water current.
From Sharks of the Atlantic Research and Conservation Coalition (ShARCC):
Some species are able to detect as little as one part per million of
blood in seawater.
... however, it is suggested that sharks are not stimulated by mammal blood (e.g. from humans) in the same way as they are by fish blood.