Tobacco smoke contains a large variety of toxic substances including several known carcinogens, respiratory irritants, cardiovascular toxins. Two varieties of tobacco smoke are mainstream smoke, which is directly inhaled and harms the smoker, and sidestream smoke, which harms those nearby who are exposed to its irritant effects on respiratory and conjunctival mucosa or obliged to breathe it. If tobacco had first been marketed after the mid-20th century, it would never have passed mandatory premarketing toxicity tests that existed by that time. Beginning in the last quarter of the 20th century, local ordinances in the United States and national laws in many other Western nations restricted smoking in public places, reducing or eliminating previously ubiquitous cigarette smoke in restaurants, etc., that was a health risk for chronically exposed workers. See also involuntary smoking.