A colourless, odorless, tasteless gas that is about one and one half times as dense as air under ordinary conditions of temperature and pressure. It does not burn, and under normal conditions, is stable, inert and nontoxic. Although it is not a poison, it can cause death by suffocation if inhaled in large amounts. Carbon dioxide occurs in nature both free and in combination. Because it is a product of combustion of carbonaceous fuels (e.g., coal, coke, fuel oil, gasoline, and cooking gas), there is usually more of it in city air than in country air. Some sources indicate that the natural balance of carbon dioxide in the atmosphere is growing from its stable level of 0.13% to a predicted 0.14% by the year 2000. It is anticipated that this extra carbon dioxide will fuel the greenhouse effect, warm the atmosphere, and further disrupt the natural carbon dioxide cycle (see Global Warming).