They're mostly water, though they can also contain impurities, e.g. "sulfur compounds (0.05% by weight in jet fuel)" [1].
More to the point: someone suspecting they're trails of pollution isn't unreasonable at first glance. Believing they're a secret government program to put toxic chemicals in the air is stupid. (I'm not ruling out governments poisoning their populations. Just that this particular way of going about it is incredibly stupid and obviously discoverable.)
They're mostly water, though they can also contain impurities, e.g. "sulfur compounds (0.05% by weight in jet fuel)" [1].
More to the point: someone suspecting they're trails of pollution isn't unreasonable at first glance. Believing they're a secret government program to put toxic chemicals in the air is stupid. (I'm not ruling out governments poisoning their populations. Just that this particular way of going about it is incredibly stupid and obviously discoverable.)
[1] https://en.wikipedia.org/wiki/Contrail