DDT and helmets
The problem with DDT is that it is a cumulative toxin, which will persist in the soil for 2 - 15 years. This means that it will inevitably build up through the foood chain. It also gives it plenty of time to work its way into the water table. Here the problem is magnified, as it persists significantly longer and is extremely damaging to aquatic life.
Although it is very effective against mosquitos that carry malaria, one WHO study linked it to an outbreak of bubonic plague in Malaysia! It's not effective against cockroaches, which accumulate it in their bodies. They are eaten by geckoes, which are poisoned and die. Cats eat the geckoes and are also poisoned and die. With no cats to control the rats, their population multiplies and they spread the plague.
There is no doubt that malaria is a serious health problem, but DDT is like cracking nuts with a sledgehammer: even if you are really careful, you may end up choking on bits of shell.
The use of cycle helmets coincides with the perceived increased risk of death or injury from more carss on the road. Some people respond by wearing a helmet, some by abandoning the bicycle. It's fear that stops people cycling, not having to wear a helmet.
I bet penguins don't like DDT.