Blog Post

Toxicological Riddles: The Case of Boric Acid

Even water is toxic if you have too much. How do we keep a potentially harmful but necessary nutrient like boric acid at safe levels in our buildings and our bodies?

We've been using boric acid and borate compounds in products for generations, praising its green virtues. The full story is a bit more complex.

What do you do about a substance that is a biologically necessary trace nutrient, long considered nontoxic, and in a multitude of products--but that is also now listed on a major European Union chemical hazard list due to evidence that it is toxic for reproduction?

It's one of those riddles that I can imagine toxicologists love to play with but that drives everyone else crazy. Here's the story, and our approach to answering the riddle--for now.

REACH and the case of boric acid

In the summer of 2010, boric acid was added to the European Union's REACH SVHC Candidate list (to spell it out: REACH = Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals and SVHC = Substances of Very High Concern). At BuildingGreen, we look to REACH as a bellwether for the direction U.S. toxic chemical regulation is eventually headed, so if something new is listed, we pay attention. Boric acid has been found to meet REACH criteria for classification as toxic for reproduction.

Huh? We've been using boric acid and borate compounds in products for a long time--in cosmetics, cleaning products (the laundry staple, Borax), as a 'non-toxic' pesticide and as an ingredient in building products. You find it in cellulose insulation, bamboo treatments, mattresses, termite protection systems, treated lumber, paints and coatings, and elsewhere. Furthermore, boric acid is a vital component to cellular structure in plants and a limiting micronutrient in agriculture. That means we spread it around on fields and elsewhere, We're eating it all the time and we have been for years.

But hazards aren't all-or-nothing

This issue illustrates why hazards aren't an all-or-nothing issue for toxicologists. Toxicology characterizes risk by evaluating:


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  • the hazard--hazardous properties of the chemical or material by itself;
  • the likelihood of exposure--who might get exposed, and how--by what route and mechanism (inhalation of VOCs or dust, ingestion, or by absorbing it through the skin, for example); and
  • the dose response--what happens when the amount of exposure is small? Or large? Or at a particular stage of life?

How to evaluate these three things, independently and together, keeps toxicologists employed and makes the rest of us a little uneasy.

How about just keeping toxic stuff out?

Green chemistry--and Pharos--tend to take a different approach, noting astutely that the more we keep hazards out of products to begin with, the less the rest of it matters and the safer we are.

For some hazards, such as Persistent Bioaccumulative Toxic Chemicals (PBTs), we at GreenSpec think this is the only sensible approach. That's because these substances stick around for a long time and accumulate up the food chain, so exposure is basically inevitable and increases as long as the stuff is produced; you can't keep anything contained indefinitely. This is also a useful precautionary approach in general and a good philosophy for developing safer chemistries, safer products, and a healthier world--but it's not the only approach needed.

What do you do if a hazard is also a nutrient, depending on the dosage, and it's neither bioaccumulative nor persistent? This is where toxicology really shines, and this is the case with boric acid and borate compounds. It's said that any medicine can be a poison, depending on the dosage--which is a different way of saying the same thing. The trick is determining where it switches and making sure we're not getting too close to that threshold.

Putting borates in perspective

In the case of borates, here are some data points.

  • Boron is a critical micronutrients in plants.
  • Most common foods contain borates at concentrations of 1–20 ppm, so we ingest up to 1 mg/day in our normal diet.
  • The tolerable upper intake level for developmental toxicity associated with ingestion of borates, according to the European Food Safety Agency (EFSA) and others, is 10 mg boron per day for an adult and 3 mg boron per day for toddlers.
  • As I said earlier, borates are all over consumer, cosmetic, building, and agricultural products. (No, that in itself doesn't make them safe--lead used to be everywhere too.)

What we do with this information

Our resulting policy for GreenSpec is as follows:

  • Don't worry about low concentrations of borate compounds in products. It's simply not a problem.
  • Carefully scrutinize higher concentrations of borate compounds, particularly where they may show up in high enough quantities in dust to cause problematic levels of exposure to workers or building occupants (For example, borates can be up to around 20% of the weight of cellulose insulation).
  • Consider the alternatives and don't freak out. While recognizing the concern that exists, we'd be horrified to see a new consumer fear of borates lead to manufacturers substituting a poorly understood PBT in its place.

As we've typically done, GreenSpec will continue let you know when borates are a major known ingredient in products. We're not likely to exclude any products from GreenSpec based on this issue, but there may be some products for which we raise a warning flag.

Avoiding dust

Cellulose insulation, for instance, is one area where significant dust during installation and remodeling could be problematic--or it might not be. A NIOSH study from 2005 concluded that the amount of respirable dust from cellulose insulation installation is typically low (there may be a lot of dust but most of it isn't the right size to get into the lungs), and so they concluded additional studies on lab animals weren't needed. The gist was that installation dust can cause eye and nasal irritation, but isn't going to create lower respiratory conditions. However, most of that respirable fraction is flame retardants, and the eye and nose irritation may be from these additives. Additionally, the study made clear "the animal pulmonary toxicity studies and worker health surveys focused on acute [cellulose insulation] exposures and do not preclude the possibility of toxicity resulting from chronic exposure. The GreenSpec team hasn't uncovered any study that gives us confidence that chronic exposure truly is or isn't a concern, but we'll keep looking.

After a dusty remodeling project is cleaned up, though, it's hard to imagine how borate compounds in cellulose insulation could be a concern. According to the U.S. EPA Exposure Factors Handbook, adults ingest on average 0.56 mg/day of house dust, so it would be virtually impossible to ingest enough dust to get anywhere approaching the concern level for borates (note that we're worrying about ingestion not inhalation here because most of the particles are trapped in the "nasopharyngeal region" --that part of your throat behind the nose--and gets swallowed, so odd as it may seem it still counts as ingestion not inhalation).

Also keep in mind that any possible concerns are still nothing like the problems with brominated flame retardants (which are PBTs) that you find in some other insulation and that dust itself can be a workplace hazard. In short, it doesn't make sense to switch away from cellulose insulation to avoid borates when it's still one of the most benign flame retardants out there, but it does make sense to take precautions regarding dust creation and exposure and seek installation methods and contractors that minimize dust. That's good precaution all around.

Given the REACH listing, we're not going to be as wholly positive about borates being nontoxic as we used to be, since there are concerns. But borates aren't volatile, aren't easily absorbed through the skin, and don't bioaccumulate the way PBTs do. So if we don't see a way that enough of it's going to be inhaled and/or ingested, it's really not a huge concern.

How borates score in Pharos

In Pharos, borates are now flagged because they're on the REACH SVHC Candidate list. But like GreenSpec, Pharos makes a distinction between PBTs--for which there really is no reasonable "exposure" argument--and other hazards.

Presence of PBDE will automatically get spray-foam insulation a ToxCon score of 1 in Pharos--the worst possible score. In contrast, the presence of boric acid in cellulose insulation will give it a score of 3. This is a low score to represent the presence of the hazard but differentiates the level of concern relative to PBTs. You can also dig deeper with Pharos and see the percentage of borate compounds in the product. If it's a trace ingredient, it's probably not worth worrying about.

Between Pharos and GreenSpec, we'll do our best to ensure that you know how to evaluate the issues from all sides and get the data you need to make up your own minds.

Published March 28, 2012

(2012, March 28). Toxicological Riddles: The Case of Boric Acid. Retrieved from

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March 19, 2018 - 8:09 am

my brother worked at a "green" insulation manufacturing plant. He states that the all boric (acid) type keeps away bats and mice. He states that it burns the feet of these critters and they can sense it and will not nest or stay in treated area. You have to use the ALL BORIC , the acid is what works

May 28, 2012 - 11:18 pm

Boric acid, 98% is sold as an effective insecticide for cockroaches, termites, fleas, silverfish and many other insects.  It's toxicity (median lethal dose, LD50) in mammals is less than table salt (Merke Index).  Yet, as documented in Wikipedia, "Although it does not appear to be carcinogenic, studies in dogs have reported testicular atrophy after exposure to 32 mg/kg bw/day for 90 days. This level is far lower than the LD50.  According to boric acid IUCLID Dataset published by the European Commission, boric acid in high doses shows significant developmental toxicity and teratogenicity in rabbit, rat, and mouse fetuses as well as cardiovascular defects, skeletal variations, mild kidney lesions. As a consequence, in August 2008, in the 30th ATP to EU directive 67/548/EEC, the EC decided to amend its classification as reprotoxic category 2 and to apply the risk phrases R60 (may impair fertility) and R61 (may cause harm to the unborn child)." 

Yet, it contiues to be effectively used in medicine, in manufacturing, in various other applications, even in dilute solution as eyewash.  Please consider all of the adverse effects of the things that boric acid fights, which would you rather have, the disease-bearing insects or the boric acid; the filth or its cleansing capability?.  Is there some in cellulosic ceiling tile?  Great!  That's the stuff for me!

BTW... do they apply the risk phrases to table salt?  In high enough doses, it may impair fertility and cause harm to the unborn child.  The matter is all in the dose!

REACH risk management process focuses on ecotoxicity and human health toxicology, and draws conclusions from logic that defies reason. 

March 30, 2012 - 2:31 am

I have been trying to find out if borate treated cellulose will deter if not stop rodents from nesting in it- as dense pack in wall cavity or loose fill in attic. Got any evidence one way or the other?

March 29, 2012 - 1:42 pm

Very interesting article. The topic of toxicology and "red lists" of chemicals to avoid is certainly gaining alot of attention. Articles like this are good to read to keep things in perspective. Thanks