Imagine walking into a library so large that no one knows exactly how many books it holds. New titles arrive every day, faster than the librarians can read, store, or catalogue them. Some books have been carefully studied. Others only partly skimmed. Many have never been opened at all. Yet, readers are already taking them off the shelves and using them in everyday life. Some may are harmless. Others contain serious mistakes or hidden dangers that no one has spotted yet. 

The modern chemical world looks a lot like this library. Over the past century, scientists and industry have created an enormous number of synthetic chemicals used in medicine, agriculture, cosmetics, and countless, everyday products. Many are very useful. But for a large share of them, we do not fully know how they behave in the environment and in our bodies.  

There is a growing gap between the number of chemicals we create and the number we can properly study to understand how they behave. This is what scientist call the chemical space problem.

A library growing out of control 

The problem is not just that the library is incredibly large. The problem is that it keeps growing faster than anyone can keep track of. New books arrive long before the old ones have been fully read, checked, or catalogued. In the chemical world, this is what happens when new substances are created and used far more quickly than scientists and regulators can study them to understand whether regulations are needed. Databases such as the CAS Registry already contain hundreds of millions of known chemicals, yet only a small fraction have been thoroughly examined for their long-term effects on human health and the environment. 

Every year, new substances enter products, ecosystems and ultimately human bodies. Modern chemistry is very good at designing molecules for specific purposes. For example, to make stronger materials, more effective medicines, and new technologies. But understanding how each substance behaves over time is much slower. Testing takes time, and regulation can only examine a limited number of chemicals in depth. 

The result is a growing gap between the number of substances in use and the number that are well understood. Large parts of the chemical world therefore remain only partly explored. The shelves keep filling, while the catalogue stays behind. 

The study of chemicals 

Even if no new books were added to the library, understanding the ones already on the shelves would still be a huge task. Reading a single “book” in the chemical world means much more than simply identifying a substance. Scientists need to find out how long it takes to break down naturally, how it moves through air, water and soil, whether it builds up in living organisms, and whether it affects our health. 

That takes time. Studying one chemical properly can take years of laboratory research, environmental monitoring, and safety testing. And even then, the picture is often incomplete. A book may be read from start to finish but not be fully understood. In other words, the challenge is not only that the library is growing too fast. It is also that every single book takes a great deal of time and effort to understand properly. 

Mixtures, interactions and new compounds 

But even that is only part of the problem. In the real world, the books in this library do not sit neatly apart, each waiting to be read on its own. They are piled together and sometimes even rewritten after they have already left the shelf. Chemicals rarely appear one at a time in the environment. Instead, they are found in combinations in rivers, soils, food, household products and the human body. This matters because the effects of a mixture can differ from the effects of each chemical on its own. Reading one book tells you something, but reading several at once can create a very different story. 

Safety testing, however, typically examines chemicals one-by-one. Real life is messier than that. For example, pesticide mixtures in surface waters can harm aquatic life even when each individual substance remains below the safety limit. In other words, the risk does not always lie in one clearly dangerous book, but in the way several seemingly harmless books interact when they are read together. 

In addition, the story can change over time. Once chemicals enter the environment, sunlight, microorganisms and chemical reactions can transform them into different substances. These will behave differently from the original chemical. Some move more easily through water or soil, while others persist for longer. So, in this library, books do not just sit beside each other, they shape each other’s meaning. They can also gain new chapters after they leave the shelf, turning into stories the librarians never expected to find. 

Use first, investigate later  

In this library, books are often already being borrowed, shared and used before the librarians have had time to read them properly. By the time someone discovers that a book contains serious errors or harmful advice, it may already have circulated widely. Substances are often used on a large scale long before scientists fully understand how persistent they are, how widely they spread, or what they may do to human and environmental health. 

That is not because no one is paying attention, but because the evidence takes years to build. Harm may only become visible after long-term exposure and regulation usually moves more slowly than innovation and commercial use. By the time concerns become strong enough to trigger restrictions, the chemical may already be present in rivers, food, wildlife and human bodies. 

This has happened repeatedly over the years. Chemicals such as PCBs, PFOA and other PFAS are widely used in a great variety of household products, like cookware, electronics and building materials. These practices continued for years before their persistence and risks became fully clear. In other words, the real problem isn’t the books themselves, but a system in which books are already in circulation before anyone has had the chance to properly read and review them. People and ecosystems can therefore be exposed long before the story is properly understood. 

Conclusion  

The chemical space problem is more than a scientific puzzle. It is not simply a matter of chemistry being complicated. But more of society producing and using substances on a scale that outpaces its ability to judge their long-term consequences. The benefits of innovation are often visible immediately, in better materials, more effective medicines and new technologies. The risks, however, often emerge slowly, after years of use and exposure. By the time those risks become clear, the chemicals are already spread everywhere. 

The central problem is not simply that there are too many chemicals. It is that uncertainty is built into the way the system works. Chemicals are often produced, distributed and used before they have been properly understood, and the burden of that uncertainty is then carried by ecosystems, public health and future regulation. We are not only living in a world filled with substances we do not fully understand. We are continuing to add new ones, faster than knowledge can catch up. 

This article was written by Joris SmitsExternal link.

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