Phenolphthalein – inhalable fraction. Documentation of proposed values of occupational
exposure limits (OELs)
Authors: Konieczko Katarzyna 
Published: December 2018

Phenolphthalein is a colorless and odorless crystalline solid; in a powdered form white or pale yellow. It is non-volatile, practically insoluble in water, but it dissolves in ethanol. Phenolphthalein is not known to occur as a natural product. The synthetic substance is used as a pH indicator in laboratories, during work on metal surfaces in galvanizing plants as well as for measuring the saturation of concrete with carbon dioxide. Until the end of the 20th century, it was widely used as a component of non-prescription laxatives – in 1999 FDA removed phenolphthalein from the list of substances considered safe. In 2016 in Poland 255 enterprises were reported to work with phenolphthalein (mainly laboratories) and there were 2500 occupationally exposed people.
Phenolphthalein used in therapeutic doses was well tolerated. Only a few side effects were reported: abdominal discomfort, nausea, reduced blood pressure and weakness. Chronic use of phenolphthalein resulted in widening of the colon, reduced thickness of the lining of the mucosa, gastric disorders, dehydration and electrolyte imbalance. In a 13-week study in which phenolphthalein was administered to laboratory animals with diets, mice turned out to be a more sensitive species from rats. Changes in testes and epididymides were observed in males and hypoplasia and bone marrow necrosis in males and females. The results of genotoxicity studies indicated that phenolphthalein acts as a promutagen and exerts a clastogenic effect after metabolic activation. Studies on the effect of phenolphthalein on the reproduction of animals indicated its harmful effect on reproductive functions of males. In the EU, phenolphthalein is classified as a category-2 mutagen and category-2 reproductive toxicant (due to its effect on fertility).
A small increase in the risk of colorectal cancer and ovarian cancer was observed in case-control studies in patients using phenolphthalein-based laxatives (especially with intensive use of these agents), but the relationship was not statistically significant. In a 2-year NTP carcinogenicity study a significant increase in the number of benign phaeochromocytomas and adenomas of renal tubular epithelium was observed in male rats. There was also a significant increase in histiocytic sarcomas in mice of both sexes and in malignant lymphomas (of all types) and thymic lymphomas and benign ovarian tumors in females. Based on these experiments phenolphthalein has been identified as a substance reasonably anticipated as human carcinogen (NTP R). The experiment on heterozygous p53 (+/-) mice of both sexes confirmed an increase in lymphoma cases. Phenolphthalein is classified by European Union experts as a category-1B of carcinogenic substances, i.e. known or presumed human carcinogens, however the classification is largely based on animal evidence. The European Chemicals Agency (ECHA) identified phenolphthalein as a substance of very high concern (SVHC).
Based on the NTP test results, the additional risk of malignant lymphoma at 8.25 mg/m3 occupational exposure to phenolphthalein for 40 years is 10-4. A concentration of 8 mg/m3 was proposed as the MAC-TWA value for phenolphthalein. Since phenolphthalein is a poorly water-soluble solid, only dust exposure of the substance will occur in the work environment, hence the proposed MAC value should concern the inhalable fraction of the substance. It is proposed to label phenolphthalein as "Carc. 1B" indicating that phenolphthalein is a category-1B carcinogen and "Ft" due to reprotoxicity. There are no bases for establishing the short-term exposure limit value (STEL) and the limit value in biological material (BEI).