Sir, - Michael Smith (August 25th) sadly undermines An Taisce's credibility by associating his organisation with propaganda campaigns relating to alleged PVC defects.These claims have been thoroughly rebutted by a substantial body of independent scientific research. The conclusions are unequivocal: 1. PVC does not contain dioxin. 2. Disposal of PVC leads to dioxin formation only to the extent associated with organic materials including wood, coal and paper. In all these cases levels of dioxin formation pose no risk to public health.PVC is low-cost, durable and versatile, properties which have recommended it to home owners, builders and architects. Other beneficial properties have established PVC as a leading material in the health-care sector, where its use contributes enormously to the comfort and welfare of patients - hardly the attributes of a toxic material as An Taisce would have us believe. - Yours, etc.,REG McCABE,Director,Plastic Industries Association,Lower Baggot Street,Dublin 2.