They can power the devices the ancient beings have used, create fast-growing food (Elemental Crops), slake the thirst (Crystals filled with Water Element), light up the darkness, or hide something in shadows (Light and Dark Crystals) or heat things up (Fire Crystals). However, the dose makes the poison, as the saying goes. Whenever the living being is exposed to the influence of the elements, it changes very slightly. If the contact is very short with a long break between exposures, the organisms will get rid of the elements inside of them, just like they would do with most of the toxins. The problems start to show up if the dose is high or the exposure is very long. In the first case, a high dose of Elements in a short period of time will most likely lead to the death of an organism, as its body will not be able to absorb so much of them. Additionally, their corpse will show which Element killed them, for example, someone who died of Fire will burn, but if they died because of Water, it will start to flow out of their orifices as if they drowned. Such a high dose could only be reached only when someone breaks one of the bigger Elemental Crystals or is surrounded by them. If the exposure to the Element is long or repeats often enough for the organism to get rid of it completely, the permanent changes start to show up. That is what is called an Elemental Sickness.
Often, it starts by changing the color of hair, fur or flowers of the affected organism. Later, more radical changes start to show up, such as the change of the texture of the skin, changes in the shape and the size of the body parts, growth of additional parts (working arms/legs/tail, feathers, scales, horns). Elemental Sickness rarely affects the insides of the organism, and those cases are exceptions.
In the first stages of the condition (change in the color of the hair/fur), it is possible to reverse those symptoms by avoiding further exposure to the Elements. Any later changes are irreversible as for now.
People born in the areas affected by the constant influence of Elements are way more resistant to the Elemental Sickness. There are specialized hazmat suits, that make their wearer completely immune to the Elements, however, they are very expensive. Normal people can reduce the dose of the Elements their body takes in by carrying a few Collectors' Crystals (there are sets of small Crystals sold specifically for that reason) or walking around with Homunculus, as those things can absorb the Elements instead of them. It is also possible to surround the area with the Crystals filled with the Element opposite to the one natural to it and neutralize them. This will last only as long as the Crystals still have some juice inside of them. It is also worth to note that staying in the area completely devoid of Elements for a long time negatively affects the psyche of humans.
Most of the people don't care if anyone has Elemental Sickness. Some of the cultures (a lot coming from the Deep) think that having the symptoms of the Elemental Sickness is a standard of beauty. There are of course those that think this condition is a punishment from gods, and anyone who has it is an abomination that should be killed.