【正确答案】正确答案： Tracking Toxic Chemicals The fireproof chemicals used in furniture and clothing / can become toxic pollutants when winding up in the environment. / As they are in the environment all over the world, / tracking the spread of them has been a major challenge. / But it got easier since a new technique has been developed / to measure the presence and concentration of these chemicals in the air by sampling tree bark. / A tree's bark provides a large surface area that takes in chemicals. / Also, because a layer of bark remains on the tree for years before being shed, / it provides a unique record of the environment over time. /