What is natural ? Here is my definition.
The essence of natural is that of the element of "cannot-help-it".
uncontrollable, unavoidable, inevitable.
With this you can explain many things.
Thus a natural event is an event just defies intervention.
The Tsunami and hurricane are natural. No one can stop it.
For human to die is natural you cannot avoid it. If human develop gene of "immortality", it will become natural to be able to live forever. When a tree in a remote area unknown to us falls. To you, it fell naturally.
Physics, Chemistry and Biology follows certain Laws that is very very strict. They are undefiable and thus natural.
For the "artifical" food people produce, it is done with human intervention.
They can help it. They can insure the food only have 10% Vitamin C added.
Even if they use natural raw material like wild oranges, they can artifically or unnaturally impose a quality control to use only wild oranges with high VitC content of more than X mg per volume.
But if you live in a society that only provide orange-based food-tablet as the only choice of food, you are given no choice. You will naturally consume the food-tablet through your mouth. And you may also naturally develop disease due to the lack of other vitamin C, and your body's natural need.
As for you poo, you cannot will your body to process the poo in a different way, so it is produced naturally. When you have got to move your bowel, you have got to move your bowel.
However human rights group concerned of your well being. Will and can interfere with the working of your society. They will recognise your unnatural situation and save you from it.
On the other hand, they may also think that you are such a wimp to not rebel against your tormentor and decide not to help you. It that case, your genes just cannot help it, to be eliminated naturally, as a matter of natural selection.
To pour toxic fumes from factory into the air, is unnatural.
This is unnatural because the people can make a decision not to do it.
It is unnatural because the toxin fumes can naturally change the current atmospheric environment and our well being.
But after it was put into the air, you cannot change history anymore.
It become natural to need to deal with the negative impact of the change in our "Natural" environment.
And but the way, if the fumes have an impact on how the sunset might look like. The definition of a natural sunset on earth have changed.
Some time we just cannot help it.