This lack of data could be for a number of reasons. The data could exist somewhere and be unavailable to certain groups as the result of intentional campaigns to hide or discredit scientific knowledge about environmental hazards. For example, ExxonMobil and other oil companies conducted extensive studies in the 1970s and 80s that confirmed the role of burning fossil fuels on climate change, but refused to release the studies and publicly denied that fossil fuels impacted climate change. Greenwashing, or the practice of portraying a company or product as environmentally friendly when it is not, is a variation of this type of data injustice.
The data also simply might not exist because the problem has not yet received enough attention or is challenging to study in some way. For example, Per- and Polyfluoroalkyl Substances (PFAS) were only discovered to be hazardous relatively recently, so there are still large gaps in our knowledge about them–including how many people in the US have been exposed, how to remove PFAS from water, and the extent of their health effects.