The Family Tree of Chemistry

Martin Mulvihill, PhD
Even though these elements are everywhere, they are almost never found in their pure form; instead they are combined to form the chemicals and materials that we are interested in using.

The periodic table is beautiful. I have no fewer than three in my office, and you can also find one in the front or back of almost any chemistry textbook. If you want to understand chemistry, it is worthwhile to spend a few minutes with the periodic table.

I will spare you the details of its many uses and jump right to the part that folks in the building industry might care about. Of the 118 elements found on the periodic table, about 98 of them are naturally occurring, and almost all of these can be found somewhere in the built environment. More than 20 of them are found in your body and are essential for human health.

“Of the 118 elements found on the periodic table, about 98 of them are naturally occurring, and almost all of these can be found somewhere in the built environment.”

Even though these elements are everywhere, they are almost never found in their pure form; instead they are combined to form the chemicals and materials that we are interested in using. For now, I will focus on three categories of materials that are assembled into products found in the building industry and explain how someone interested in materials health might approach these categories. In future posts I will talk in more detail about each of these categories and where to find the best information about their safety.

Category 1: Small Molecules—like solvents, dyes, plasticizers, and surfactants—are what people typically think of as chemicals. Small molecules are relatively easy to classify by CAS number, name, or chemical structure. Although a lot more information is needed about many of these molecules to accurately assess their safety, we often have the tools and knowledge to produce this information (assuming someone is willing to pay for the testing).

Some types of small molecules are suspected to cause health problems when consumers are exposed to them in products. Phthalates, BPA, parabens, and benzene are all small molecules that have been linked to negative health effects in consumers. In addition to the concerns about certain chemicals found in products themselves, many additional small molecules are used during manufacturing and processing. Sometimes chemicals used upstream in the supply chain create hazards for workers and the environment. For example, hexane that was being used to clean iPhone screens sickened Chinese factory workers even though it wasn’t part of the final product.

Category 2: Polymers and Plastics are some for the most common materials found in the built environment. All of our carpet, wall coverings, and many coatings (such as paint and varnish) are based on polymers and plastics. Wood is an example of a natural polymer-based building material. (Wood is actually a composite of three polymers: lignin, cellulose, and hemicellulose all held together by other polymers and small molecules including starches, proteins, and fatty acids.)

Polymers are large molecules made up of repeating units (called “monomers”), whereas plastics are the final product once all of the dyes, plasticizers, fillers, and other additives have been combined. Polymers themselves are generally not harmful in their final form. Most health concerns associated with plastics come from their constituent monomer (like BPA used to make polycarbonate plastics) or from the small molecules that have been added, such as phthalates in PVC. Unlike small molecules that are easy to classify and evaluate, plastics are much harder because they are a combination of many different components. There are also other issues to consider when evaluating the overall health impact of plastics, like what sort of feedstock they come from, whether they are easily recyclable, and what happens to them at the end of life.

Category 3: Metals and Ceramics are both based on elements found in the middle of the periodic table and are found in a huge variety of products. Metallic elements are often found incorporated into products as alloys or ceramics. Alloys are mixtures of a metal and one or more other elements (usually other metals) that have been combined to give particular properties. For example steel, bronze, and brass are all alloys. The chemical structure of ceramics contains both metallic elements and non-metals, like oxygen or sulfur. Dry wall (gypsum board), cement, and titanium dioxide additives used in paints and other products are all examples of ceramics.

Metals and ceramics occur in many forms, shapes, and sizes, which influences their material properties and determines their health effects. For example, the chromium used in stainless steel alloys is safe for food contact applications like knives, while the chromium dyes in oil-based paints are potentially very toxic. The chromium in the knife blade is different from the chromium in the dye in two important ways: (1) the chromium in the alloy is bound very tightly to the iron and carbon that make up the rest of the blade; and (2) the chromium found in the knife blade is in a safer form (Cr metal or Cr III), while the chromium in some dyes is (Cr VI), which is the toxic form made famous by the movie Erin Brockovich.

The health effects of ceramic particles, like titanium dioxide, can depend on their size and shape. While larger particles of titanium dioxide may be safe, smaller particles that can disperse in air and be inhaled readily are probable carcinogens. The very small (more than 10x smaller than the diameter of a human hair), needle-like nature of ceramic mineral asbestos fibers is what allows them to be breathed in and causes them to remain lodged in our lungs, contributing to long-term toxic effects.

Because the hazards vary depending on certain attributes of metals and ceramics, it can be challenging to find relevant and reliable toxicity information. In these cases, a simple ingredient name without additional information does not provide enough information to determine human health hazards. While it is a good idea to avoid highly toxic materials like lead and mercury whenever possible, it is also true that you will need to consider how ingredients are being used, and if they are in a form that could be inhaled, ingested, or absorbed through the skin.

These three types of materials—small molecules, polymers, and metals and ceramics—are all made up of the elements of the periodic table arranged in different ways. This diversity of materials is what makes chemistry so versatile and allows chemists, manufacturers and product designers the flexibility to create such a wide range of products. Understanding the differences among these categories of materials will enable you to ask better questions about material health and get better answers.

My colleagues and I will explore each of these categories in future posts, including a special post about bio-based materials. Stay tuned!

Martin Mulvihill, PhD
USGBC Senior Research Fellow, Executive Director of the Berkeley Center for Green Chemistry (BCGC), and researcher in both Public Health and Environmental Engineering. Ph.D. from the University of California, Berkeley in Chemistry and Nanoscience.