More employers are making workers pay more for health insurance if they smoke, are overweight, or have high cholesterol.

About 20 percent of major American employers (those with over
1,000 workers) have policies that impose financial penalties on employees with certain habits or health conditions -- twice as many as there were just two years ago, according to a recent survey. The practice is expected to double yet again next year, according to Towers Watson, the benefits consultant firm that conducted the survey.

Walmart is one such employer, having recently added a $2,000-per-year year surcharge for its employees who smoke. The company has also started an antismoking program, and about 13,000 of its more than 1 million employees have enrolled. Other companies currently financially penalizing its workers include Home Depot, Lowe's, PepsiCo, Safeway, and General Mills

Meanwhile, another survey released this week shows that one-third of employers with 500+ workers are trying to win over employees to healthier behaviors with discounts on their insurance.

Organizations such as the American Cancer Society and American Heart Association are concerned that such policies could open a "back door" to discrimination against unhealthy workers.