Rockwool is a sterile, porous, nondegradable material manufactured in a variety of forms for use as home insulation and as a growing media in hydroponic gardening. It was developed in Denmark in the 1970s, but with its increased popularity in the United States, it has come under recent scrutiny following claims that its fibres can be harmful.
Natural basalt rock and chalk are melted into lava at 1,600 degrees Celsius. The molten material is then blown into a large, spinning chamber, which transforms it into long, fibrous stands, much like candyfloss. It is an eco-friendly process that produces a material that is chemically and biologically inert and clean of plant pathogens.
The dust from rockwool can be a skin irritant. George F. Van Patten and Alyssa F. Bust, authors of "Gardening Indoors With Rockwool," advise handling rockwool with gloves, keeping it away from children and animals, and washing clothing after working with it.
Although it can never hurt to use caution, it is important to know that rockwool fibres are single monofilament strands and, unlike asbestos, they do not split into ultra-thin fibrils that can penetrate cell walls within the body. Rockwool fibres are short and thick, so the body can easily discharge and dissolve them.