Vented tumble dryers release moist warm air from the drying process to the external environment, usually through pipework linking the appliance to a vent in an exterior wall. Although such dryers contain a lint filter to remove fibers from this air stream, recent reports suggest that this process is incomplete, leading to microfibers being released in the ducted warm air and subsequently polluting the external environment. Microfiber release from wash loads comprising 10 100% cotton and 10 100% polyester T-shirts (total load mass ratio 48% cotton, 52% polyester) was measured at different stages of the washing and drying process to compare the quantities of fibers released 'down the drain', collected in the dryer lint filter, and released to air from the tumble dryer. Testing under both European and North American washing conditions found that the quantities of microfibers released to air during tumble drying were significant and comparable to levels released 'down the drain' during washing. Use of conventional rinse-added liquid fabric conditioner increased microfiber accumulation on the dryer lint filter, with reduced release from the dryer exhaust observed at the highest fabric conditioner dose tested (21.6% and 14.2% reduction under North American and European conditions, respectively). Conventional liquid fabric conditioner did not significantly impact microfiber release from the washing machine, in line with previous studies. A fabric conditioner specially designed for anti-wrinkle performance reduced microfiber release from the dryer exhaust at all levels tested (by 17.6-35.6%, depending on dose), apparently by increasing the efficiency of microfiber accumulation in the lint filter. Tumble dryer sheets were also found to cause a reduction in microfiber release from the dryer exhaust (by 14.1-34.9%, depending on the dose/product), likely driven by collection of liberated fibers on the sheet during the drying process. The use of both antiwrinkle liquid fabric conditioner and dryer sheet enabled a 44.9% reduction in microfiber emissions from the dryer exhaust. In all studies, the fiber mass collected on the lint filter or emitted from the dryer exhaust was richer in cotton fibers (range 83.4-96.3% on the lint filter, 93.0-99.8% from the dryer exhaust) than the wash load composition (48% cotton). Moreover, fibers collected by the lint filter contained a higher proportion of polyester than emissions from the dryer exhaust (range 3.7-16.6% on the lint filter, 0.2-7.0% from the dryer exhaust). There is significant variation in the porosity of lint filters among installed vented tumble dryers. Single-variable testing of the impact of lint filter design concluded that reducing screen pore size significantly reduces airborne microfiber release during tumble drying; a reduction in lint filter pore size from 0.2 mm2 to 0.04 mm2 reduced release by 34.8%. As some lint filters have pore sizes of around 1 mm2, there is enormous scope to reduce microfiber release from dryers though improved lint filter design. However, it is suggested that a step-change in appliance design away from vented dryers to only fully-sealed condenser dryers might be necessary to eliminate the contribution of tumble drying to airborne microfiber pollution.