This question has been asked many times by shippers, carriers, ports and forwarders for many years and finally the answer has come through 19th Revised Edition of UN Model Regulations.
The International Table Tennis Federation rules for 40mm ball says the ball has a diameter of 40mm and a weight of 2.7 g and the ball shall be made of celluloid or similar plastics material and shall be white or orange, and matt. The variation acceptable to weight per ball is set between 2.67 g and 2.77 g and no more than 1 ball in 24 sample balls shall be outside this range. Weight less than 2.60 g or greater than 2.85 g are considered as outliers.
Celluloids are readily ignitable and when involved in a fire, evolves toxic fumes; in enclosed cargo spaces, these fumes may form an explosive mixture with air.
UN Model Regulations 19th Revised Edition which is incorporated into IMDG Code 38th Amendment classifies Table tennis balls made of celluloid as dangerous goods and same must be consigned under UN Number 2000 Class 4.1 unless, according to special provision 383, each table tennis ball is not more than 3 g net mass and total net mass of table tennis balls is not greater than 500 g per package.