Like much economic activity, the event industry not only impacts on its local environment, but also on the global environment. Some of this relates to pollutants that affect the local area before becoming more widely diffused, such as sewage, increased run-off, oil and other liquid discharges. Much of it relates to gaseous discharges. These are more mobile than solid and liquid wastes and are usually discharged directly to the atmosphere. If warmer and/or lighter than air, they rise. The ascent may be many thousands of metres. At all levels of the ascent they can be easily carried away by winds. Even heavier gases can be blown away by ground-level winds. The gases get caught up in the general circulation of the atmosphere and can be carried around the globe and throughout the various layers of the atmosphere. Various factors such as the prevailing wind directions at different altitudes can result in the gases becoming concentrated in parts of the world far from the source of the pollution. Examples of this are the chlorofluorocarbons (CFCs) that led to the breakdown in the ozone layers over the Arctic and Antarctic, and the acid rain (resulting from CO, CO2, SO2, SO3 and NO3 emissions) that has damaged forests in Scandinavia and central Europe. Other gases become universally distributed, such as the so-called greenhouse gases.