The distance to the source does not play a role in the sense of smell, only the concentration of the odorous substance. 1) "Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. They are more attracted to the chemicals found in the guts of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey." Source and further information: http://en.wikipedia.org/wiki/Shark 2) "The target of this meaningless statement which can be found in many different versions in the media, is clear. The author wishes to express the notion that sharks have a well-developed sense of smell. This in effect is correct, sharks are capable of recognizing even very small concentrations of scents in their immediate environment. However, the fact that sharks may smell blood at long distances says virtually nothing about their sense of smell, the reason being that the author sets the shark's sense of smell in relation to the distance from its source. Nothing is mentioned about the concentration of molecules necessary for the shark to notice the smell. But this is exactly the decisive point. The actual distance to the source is irrelevant. For example, humans have a significantly worse sense of smell than sharks, and yet we may still identify the smell of blood from significant distances if the amount of blood is large enough for the molecules to reach our mucous membranes (the nose) in adequate concentrations. Personally, I at least once smelled the smoke of a forest fire burning about 100 km away! But not because my sense of smell is as phantastic as that of sharks, but rather because the source of the smell (the fire) was big enough to allow a sufficient concentration of smoke to reach my nose from a distance of 100 km. The significance lies in the concentration of molecules and the decisive question can be posed accordingly: Does the available concentration of odorous substances exceed the perception limit of a shark (or a human)? This is the only way to properly discuss the perception limits of a species. All other statements on distance have nothing to do with the question and are irrelevant. So what are the facts? How effective are shark noses in reality? Sharks can recognize most amino acids, the basic component of proteins, even in such small concentrations of 10-10 molar (a "one molar" solution is equivalent to the molecular weight of a substance in grams dissolved in one liter of water). Permit me to express this astronishing sensory perception in a more understandable manner. First, it means that sharks smell about 10,000 times better than humans. Second, to reach this concentration about 30 mg (measuring half a pinch, editor's comment) of an amino acid would have to be dissolved in a large swimming pool filled with saltwater! Third, such a swimming pool would have to have a volume of 2,600 cubic meters and this amount of saltwater would contain 3.5% of cooking salt and other ions, in other words, an approximate total of about 92 tons of salt. It is very hard to imagine that a shark can still perceive these few milligrams of amino acid when mixed with tons of salt. Fourth, sharks can even sense the presence of certain amino acids better than others. For example, they smell the amino acid Serin in concentrations of only 10-14 molar! Fifth and finally, it must also be said that although such sensory perception levels may be quite astronishing to humans, for other animals they are fairly run-of-the-mill. Ordinary animals such as lobsters or catfish have a comparable if not better sense of smell than sharks. However, all comparisons with chemoreception performance such as the sense of smell must be made in relation to the concentration of the odorous substance and not the distance to their source." Source and further information: http://www.sharkinfo.ch/SI3_99e/noses.html 3) "Diffusion and Water Currents Sharks receive scent through water by two methods, diffusion and water currents. The diffusion method is very slow, and involves the blood molecules slowly dispersing through the water, spreading and being diluted with the water as it moves away from its starting point. The water current method involves the blood being moved by the movement of the water. This is the faster method for scent to travel through water. Depending on how fast the water is moving, the scent of blood could reach a shark anywhere from one minute to a dozen or more minutes, depending on how far distant the shark is. Distance Has an Effect on The Shark's Sense of Smell The only time a shark will instantly react to the smell of blood is if that shark is in the immediate vicinity of the blood spill. Most of the time it takes several minutes for the shark to smell the blood and several more minutes for it to arrive at the spot of the spill. Sharks will swim into the current where they detect the odor of blood. From a mile away this is very difficult for the shark and it will likely not find the blood scent in the first place but from a much shorter distance, say a quarter of a mile, the shark has a decent shot at picking up a strong scent. The hunting shark will swim in spirals, thereby eliminating the currents that don't carry the scent of blood and following those that do." Source and further information: http://www.ehow.com/how-does_4579345_shark-smell-blood-miles-away.html http://www.answerbag.com/articles/How-Does-a-Shark-Smell-Blood-From-Miles-Away/d4e40a63-7985-6c35-cbd0-93f4a4d42672 Further information: - "How quickly can sharks detect blood in the water?": http://www.straightdope.com/columns/read/2781/shark-youre-whats-for-dinner - "Sharks Smell Blood From A Mile Away?": http://www.thenakedscientists.com/forum/index.php?topic=10171.msg127187