At the simplest level: we need air, water and food. Our bodies are pretty capricious; they need balancing and are prone to disbalance.
Also, our culture is strangely promoting many health practices which are not good for you. One of the worst practices these days is the extreme overuse of pharmaceuticals. This is very irrational since pharmaceuticals are promoted by the industry and against the interests of patients. Patients are simply being deceived by the industry which acts selfishly. There are many more examples of irrational selfish practices of groups of people against humanity. Crime is the most obvious example of such.
The quality of air depends on where you are. America succeeded in moving all dirty industries abroad, mostly to Mexico and China. Now, these countries take a toll on ecology. In big cities, especially in poor countries, the air is polluted to high levels making people sick.
This is the same with water. The natural water in rivers and on the shore where I live is not very natural. We enjoy time on the lake, but its water is dirty near the shore from human-created pollution. (Still, this pollution is mostly agricultural, not industrial. Industrial pollution is much worse. In Russia, I have experienced much worse cases of industrial pollution of air and water with nasty chemicals. From the place in America, where I live now, it takes about 1 hour to drive (about 70 miles) to get to the place where the air is badly polluted with chemicals, which is not too bad, compared to poorer countries.
Our food becomes relatively cheaper with time, but its contamination with toxins is high. So-called "organic food" is cleaner but more expensive.
People are kept ignorant by the mainstream media about the importance of electromagnetic ecology. Human physiology requires healthy Earth's electromagnetic rhythms for proper function. Electromagnetic pollution grows much in recent years, especially with the progress in wireless communications. I use wireless devices extensively; they currently offer more good than harm. I also live near a radio tower with many transmitters. I wish my family and I could move into a cleaner place, but the move is prohibitively expensive. Our living place and way of life are quite inefficient but we are stuck in the place and way of life because of real and perceived financial difficulties.
Cell phones and wireless computers contribute much to electromagnetic pollution. Yet, computers and phones now unite people and this uniting has very positive effects on humanity. I am writing these words using a very convenient handheld device wirelessly connected to the network. This saves time and also gives me peace of mind since I can write in comfort while every text line is saved on a remote server with a backup. Empirically, I would guess electromagnetic pollution is not as bad as one of air, water and food.
Noise is another pollutant. Finding a quiet place is sometimes very difficult. I became much healthier as I learned to use noise-blocking headphones and a sleep mask for my meditations.