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My parents always told me not to eat from the apple trees near the road, because some kind of bad toxic dirt might get through.

This sounds to be physically possible but not really probable in my view. In the time apples grow on the tree they never rot. The rotting procces starts as the apple falls down and damages - the wound will quickly change its colour and it begins to decay - from this I assume the peel is almost perfect defense against bacteria & toxic penetration.

I think the only danger may come from the dust microparticles spread by the cars - which can be easily washed by water or wiped off with a piece of cloth. Am I right? What about other types of fruit?

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    $\begingroup$ Have you considered the possibility that the stuff might get dissolved by water and taken up by the tree by the roots? $\endgroup$ – Alex Sep 2 '16 at 5:58
  • $\begingroup$ Hmm... Why the irony? $\endgroup$ – Alex Sep 10 '16 at 13:42
  • $\begingroup$ FWIW, apples can and do rot on the tree, for instance if they're pecked by birds, or have been infested with codling moths. $\endgroup$ – jamesqf Dec 5 '16 at 2:51
  • $\begingroup$ it's waxy so it's a very good barrier, and the truth is that apple peel contains more good stuff that most of the inside of the apple. You can consider the peel to be the same as a leaf, perhaps a lot more resistant and waxy, so you can search for real science in a lot of research about leaves content of roadside pollutants, for example: ncbi.nlm.nih.gov/pubmed/17294646 $\endgroup$ – aliential Nov 14 '18 at 19:09
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The answer I got from Jan Kolář of Charles University:

The spores of fungi or bacteria in apples don't just emerge from nothing, they have to get into it from the surroundings. I often see apples, which rot while still hanging on the tree, the hole needed for the infection of the apple is usually made by a worm. The problem of the apples near roads isn't that they would rot. Indeed, plants can absorb all kinds of harmful substances from soil, water or air, including heavy metals, chlorinated organic compounds or organic compounds from the exhaust gases of cars. How much and which substances, that usually depends on the type of plant (specifically on where it stores most of the substances).

It's not enough to wash an apple because the impurities are absorbed in the fruit itself as well. And on the surface of the peel, there are waxes and other unsolutabe substances which can absorb toxic aromatic carbohydrates from the exhaust gases, and can't be simply washed off.

The greatest risc near the roads used to be contamination with lead because petrol used to contain it. Many of the plants can recieve the lead and hoard it up in some organs. Now, when the lead petrols are forbidden, the biggest risc pose the fumes from the diesel engines. So I'd rather not eat fruit near the roads (especially near the busy ones).

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