A collapsible ladder comprising: two vertical stiles with a plurality of rungs disposed between two stiles, each stile being formed by a plurality of telescopically collapsible columns (1a, 1b, 21, 22); wherein each or at least part of the collapsible columns (1a, 1b, 21, 22) has a bottom opening and an inner wall and receives a cover at the bottom opening to cover the bottom opening; wherein the cover has a small cylindrical portion and a large cylindrical portion with a peripheral surface located under the small cylindrical portion; the small cylindrical portion is inserted into the bottom opening of a column and is positioned inside the column, while the large cylindrical portion is exposed outside of the column and inserted inside of a next lower column; an annular groove is formed on the peripheral surface of the large cylindrical portion; and a sealing ring is disposed inside the annular groove, with the sealing ring touching the inner wall of the next lower column when the stile is extending or collapsing, said sealing ring forming a frictional seal for the next lower column and dividing the next lower column into an upper part above the sealing ring and a lower part below the sealing ring; characterized in that one or more air ducts (30a, 30b) are provided in the cover, each air duct (30a, 30b) having a first air opening and a second air opening, with the first air opening arranged above the large cylindrical portion and in correspondence with an opening in a lower wall part of the column receiving the cover, whereby the first air opening faces the inside or the interior of the upper part of the next lower column, and with the second air opening facing a bottom part of the cover and the inside or the interior of the lower part of the next lower column, thereby enabling air to exit from the upper part to the lower part of the next lower column. two vertical stiles with a plurality of rungs disposed between two stiles, each stile being formed by a plurality of telescopically collapsible columns (1a, 1b, 21, 22); wherein each or at least part of the collapsible columns (1a, 1b, 21, 22) has a bottom opening and an inner wall and receives a cover at the bottom opening to cover the bottom opening; wherein the cover has a small cylindrical portion and a large cylindrical portion with a peripheral surface located under the small cylindrical portion; the small cylindrical portion is inserted into the bottom opening of a column and is positioned inside the column, while the large cylindrical portion is exposed outside of the column and inserted inside of a next lower column; an annular groove is formed on the peripheral surface of the large cylindrical portion; and a sealing ring is disposed inside the annular groove, with the sealing ring touching the inner wall of the next lower column when the stile is extending or collapsing, said sealing ring forming a frictional seal for the next lower column and dividing the next lower column into an upper part above the sealing ring and a lower part below the sealing ring; characterized in that one or more air ducts (30a, 30b) are provided in the cover, each air duct (30a, 30b) having a first air opening and a second air opening, with the first air opening arranged above the large cylindrical portion and in correspondence with an opening in a lower wall part of the column receiving the cover, whereby the first air opening faces the inside or the interior of the upper part of the next lower column, and with the second air opening facing a bottom part of the cover and the inside or the interior of the lower part of the next lower column, thereby enabling air to exit from the upper part to the lower part of the next lower column.