Complex sentence: Evidence also shows that inhaled sidestream smoke, the main component of secondhand smoke, is about four times more toxic than mainstream smoke, a fact that was known to the tobacco industry since the 1980s, which kept its findings secret.
Split into simpler sentences: Evidence also shows that inhaled sidestream smoke, the main component of secondhand smoke, is about four times more toxic than mainstream smoke. This fact has been known to the tobacco industry since the 1980s, though it kept its findings secret.