Children as young as 13 who have evidence of secondhand smoke in their blood also have visibly thicker arteries, researchers have reported.

The study suggests that the damage caused by secondhand tobacco smoke starts in childhood and causes measurable damage by the teen years. It look at nearly 500 children aged 8 to 13 taking part in ongoing research on heart disease. The scientists measured levels of cotinine, a byproduct of nicotine that is found in the blood after someone breathes in tobacco smoke.

The children with the most cotinine in their blood had carotid artery walls that were, on average, 7 percent thicker. Their aortas were 8 percent thicker.

The researchers also did a test that measures the flexibility of the arteries in the arm, another measure of blood vessel health and heart disease risk. This measurement was 15 percent lower in teenagers with the highest levels of cotinine.