Errrr....didn’t it go ‘on tour’ for a while?
It was not allowed to be contaminated by anything from “all over the world”.
Uh, yeah, it was. In the past, it was not unusual for it to be paraded through the town where it was, being held by priests in their hands, where dirty, dusty pilgrims from everywhere were lined up along the streets quite close to it, were allowed to reach out and touch it. The streets were dusty and dirty as well. It would NOT have been washed after one of these parades and merely put away.
Robert du Clary, a knight with the fourth Crusade related how every Sunday, the Shroud would be brought up on some kind of support, out of its reliquary like a drapery in the Hagia Sophia in Constantinople so that "The Image of Our Lord stood up for all to see". . . Thus, dust and wind from a very bustling sea port with ships visiting from all over the world could have brought pollen from everywhere.
Until the 20th Century, every exposition of the Shroud was open air. . . no glass covered the Shroud as hundreds of thousands of pilgrims walked past within twenty feet or so, raising clouds of dust which would have certainly allowed some of those pollens to fall on the cloth of the Shroud during those expositions. I believe that the first glass enclosure was the 1932 exposition and that has been the practice since, because prior to that, such large panes were impractical and/or gave too much glare for the pilgrims to get a good view. Even in 1978 the scientists were aghast to learn the priests had merely displayed the Shroud on a backing of plywood using rust prone, steel, THUMB TACKS (!!) which had indeed left a ring of rust where ever they had been on the Shroud.