If something is deleterious, it does harm or makes things worse. Smoking has obvious deleterious effects on your health, not to mention your social life.
My parents were worried that their divorce would have a deleterious effect on us kids, but in the end it was less harmful than watching them fight all the time. For most plants, a lack of sunlight has very deleterious consequences, but there are some plants that actually do very well in the dark. It's a wonder, given how well-established the science is, that we continue to do things that are deleterious to the fragile ecosystem. We must be willing to live with the deleterious effects.