wind speeds. Wood smoke (for example from wildfires) can cause lung damage, artery damage and DNA damage leading to cancer, other respiratory and lung disease and cardiovascular disease. Air pollution, particulate matter and wood smoke may also indirectly cause brain damage because of particulates breaching the cardiovascular system and into the brain, which can increase the risk of developmental disorders, neurodegenerative disorders mental disorders, and suicidal behavior, although studies on the link between depression and some air pollutants are not consistent. At least one study has identified "the abundant presence in the human brain of magnetite nanoparticles that match precisely the high-temperature magnetite nanospheres, formed by combustion and/or friction-derived heating, which are prolific in urban, airborne particulate matter (PM)." Air pollution has also been linked to a range of other psychosocial problems. Measurement As early as the 15th century Leonardo da Vinci commented at length on the difficulty of assessing smoke, and distinguished between black smoke (carbonized particles) and white 'smoke' which is not a smoke at all but merely a suspension of harmless water particulates. Smoke from heating appliances is commonly measured in one of the following ways: In-line capture. A smoke sample is simply sucked through a filter which is weighed before and after the test and the mass of smoke found. This is the simplest and probably the most accurate method, but can only be used where the smoke concentration is slight, as the filter can quickly become blocked. The ASTM smoke pump is a simple and widely used method of in-line capture where a measured volume of smoke is pulled through a filter paper and the dark spot so formed is compared with a standard. Filter/dilution tunnel. A smoke sample is drawn through a tube where it is diluted with air, the resulting smoke/air mixture is then pulled through a filter and weighed. This is the internationally recognized method of measuring smoke from combustion. Electrostatic precipitation. The smoke is passed through an array of metal tubes which contain suspended wires. A (huge) electrical potential is applied across the tubes and wires so that the smoke particles become charged and are attracted to the sides of the tubes. This method can over-read by capturing harmless condensates, or under-read due to the insulating effect of the smoke. However, it is the necessary method for assessing volumes of smoke too great to be forced through a filter, i.e., from