Common Hazardous Ingredients in Personal Care Products
More than 10,000 ingredients are allowed for use in personal care products — and the average woman wears 515 of them every day, according to a 2009 British study that looked at the routines of over 2,000 women. Very little is known about the health effects of these chemicals. More than 90% have never been tested for their effects on human health, and complete toxicity data are available for only 7% of them. Even though government agencies are aware of the health hazards of some ingredients, such as hydroquinone or phthalates, they are still allowed in personal care products.
We are providing information on some of the most common hazardous ingredients, so that you can check your cosmetic labels and see if they are there. Hazardous ingredients are usually present in conventional products, but they may also be found in some “alternative” products which try to be more health conscious. Note though that some chemicals about which there are serious concerns. such as fragrance ingredients or contaminants found in certain chemicals, will not show up on labels so reading labels won’t tell you everything you need to know. The information below should be helpful.
In preparing this guide, we screened products and chose those which had the least amount of these hazardous chemicals, or none at all, for our Best and Good sections.
Alpha Hydroxy Acids (AHA) and Beta Hydroxy Acids – Alpha hydroxy acids and beta hydroxy acids are acid “skin peels” marketed as a way to remove wrinkles, blemishes, blotches and acne scars. With their use, “the skin reddens like a sunburn, then darkens and peels away supposedly leaving ‘new’ skin”, according to the US Food and Drug Administration. Beta hydroxy acid is preferred for oily skin. These skin peels enjoy widespread popularity. The Environmental Working Group found they were added to one out of every 17 personal care products on the US market. They can be found in skin care products ranging from moisturizers and cleansers to eye creams and sunscreen. The FDA estimates that they injure 1,000 Americans every year by burning the skin. The FDA is also concerned that they contribute to UV skin damage and may raise the risk of skin cancer. In Canada, the Health Canada Cosmetic Hotlist allows concentrations of less than 10% in personal care products, but higher concentrations are allowed for professional use. Health Canada also requires cautionary warnings on leave-on products containing AHA when the concentrations are above 3%.
Aluminum – Aluminum compounds are the active ingredients in antiperspirants. By temporarily plugging the sweat ducts, they stop sweat coming to the skin’s surface. A 2005 British study, published in the Journal of Inorganic Biochemistry, found that aluminum-based compounds may be absorbed by the skin and cause estrogen-like effects. Because estrogen has the ability to promote breast cancer cells, some scientists have suggested that the aluminum-based compounds in antiperspirants may contribute to the development of breast cancer. A 2003 study in the European Journal of Cancer found that women who used antiperspirants or deodorants and who shaved their underarms at an earlier age were at greater risk for breast cancer than women who started later.
Benzyl Alcohol and Isopropyl Alcohol – Both benzyl alcohol and isopropyl alcohol irritate the skin. They are used as fragrance ingredients and as preservatives, solvents and anti-foaming agents for hand sanitizers, sunscreens,lotions and baby wipes. There is also evidence that these two alcohols are neurotoxic. Children younger than 3 years old are particularly at risk for toxic effects if they are exposed to benzyl alcohol.
Boric Acid and Sodium Borate – Boric acid and sodium borate are preservatives in personal care products and baby products, which are easily absorbed into the skin. Although they are considered by the cosmetic industry to be unsafe for infants or for damaged skin, they are an ingredient in many diaper rash creams and moisturizers.
Bronopol (2-Bromo-2-Nitropropane-2,3-Diol) – Bronopol, a preservative, is a lung, immune system and skin toxicant, and has been shown to disrupt the endocrine system in animal studies. Yet, it is used in baby wipes, conditioners, liquid soaps and body washes. Bronopol can break down into formaldehyde, a known carcinogen, and into nitrosamines, which are suspected carcinogens.
Butylated Hydroxy Anisole (BHA) and Butylated Hydroxy Toluene (BHT) – Butylated hydroxy anisole (BHA) and butylated hydroxy toluene (BHT) are closely related preservatives and antioxidants. They slow down the rate at which product ingredients change colour. They are present in lipsticks, eyeshadows and many other types of cosmetics. Both BHA and BHT are skin allergens. BHA is “reasonably anticipated to be a human carcinogen” by the US National Toxicology Program. It is also used in fragrances, although this use is not allowed in the European Union because it can cause skin depigmentation. For BHT, there is some evidence that it promotes tumours and can disrupt the hormonal system.
Coal Tar Dyes – Used extensively in personal care products, coal tar colours are often identified on ingredient lists as FD&C, D&C or C.I. followed by the colour name or number. As their name suggests, they are made from coal tar, a petroleum product. Many people experience allergic reactions like skin irritation and contact dermatitis. Some evidence suggests that certain coal tar colours cause cancer — D&C Blue 1, D&C Green 3, D&C Red 4, and D&C Yellow 5. Coal tar itself is a recognized human carcinogen and is banned from use in cosmetics. However, each coal tar dye has different properties and different potential health concerns. On US products, coal tar dyes are listed as FD&C or D&C, followed by a colour and a number (F indicates that the colour is also approved for food use). In Canada, they may be identified as C.I. (Colour Index) followed by a 5 digit number or as p-phenylenediamine. (Natural and inorganic pigments are numbered in the 75000 and 77000 series respectively.) In Canada, coal tar colours are permitted in hair dyes only if the labels carry warnings about skin irritation and possible blindness if the product is used for dying eyelashes or eyebrows. Health Canada does not allow many of these colours to be in products sold for use in the area of the eye, but there are no restrictions on their use in other products.
1,4-Dioxane – Because it is classified by the US Environmental Protection Agency as a probable carcinogen, 1,4-dioxane is a particular concern in children’s and baby products. It is a contaminant in shampoos, body wash, children’s bath products and other sudsing cosmetics. Because 1,4-dioxane is a contaminant and not an ingredient, it doesn’t appear on ingredient labels. Product tests done in 2009 for the Campaign for Safe Cosmetics found 1,4-dioxane in in baby shampoos and in many bath products marketed for young children. 1,4-dioxane can form during a manufacturing process called ethoxylation. Ethoxylation uses ethylene oxide (a mammary carcinogen in animals) to make other chemicals less abrasive. For example, ethylene oxide converts the harsh sodium laurel sulphate to the milder sodium laureth sulphate (the “eth” in laureth shows ethoxylation), which can result in 1,4-dioxane contamination. In addition to sodium laurel sulphate, ethoxylation is used for many different chemicals used in cosmetics.
DMDM Hydantoin, Diazolidinyl Urea and Imidazolidinyl Urea – DMDM hydantoin, diazolidinyl urea and imidazolidinyl urea are commonly used preservatives that can release formaldehyde. Formaldehyde is a sensitizer and a proven carcinogen. Exposure to formaldehyde may cause joint pain, depression, headaches, chest pains, ear infections, chronic fatigue, dizziness and loss of sleep. It is estimated that 20 per cent of people exposed to DMDM hydantoin will experience an allergic reaction. Imidazolidinyl urea may cause contact dermatitis in some individuals.
Formaldehyde – Formaldehyde is a known sensitizer and a known carcinogen. Exposure to formaldehyde may cause joint pain, depression, headaches, chest pains, ear infections, chronic fatigue, dizziness, and loss of sleep. Formaldehyde may be used in personal care products as a disinfectant, germicide, fungicide and preservative. It can be found in soaps, shampoos, hair preparations, deodorants, lotions, shaving cream and mouthwash. it is also used in nail products, specifically as a nail hardening agent, and is one of the “toxic trio” targetted for elimination from nail polish and removers by the Campaign for Safe Cosmetics. It has also been found in high concentrations in certain hair straightening products. Formaldehyde may also be released when the preservatives, DMDM hydantoin, diazolidinyl urea and imidiazolidinyl urea are present in products, as discussed above.
Fragrance – Synthetic fragrance is the most common ingredient found on the label of personal care products. The generic terms, “fragrance” or “parfum”, can indicate the presence of up to 3,000 separate ingredients. Most or all of them are synthetic. Fragrance is a sensitizer and a known trigger of asthma. Symptoms reported to the FDA have included headaches, dizziness, rashes, skin discoloration, violent coughing and vomiting, and allergic skin irritation. A test of fragrance products by the Campaign for Safe Cosmetics, “Not So Sexy”, found that perfumes contained an average of 10 known sensitizing chemicals, which can trigger allergic reactions such as asthma, wheezing, headaches, and contact dermatitis. In addition, clinical observations by medical doctors have shown that exposure to fragrances can affect the central nervous system, causing depression, hyperactivity, irritability, inability to cope, and other behavioral changes.” (Home Safe Home, Debra Lynn Dadd). Many of the compounds in fragrance are also suspected or proven carcinogens. In 1989 the US National Institute of Occupational Safety and Health evaluated 2,983 fragrance chemicals for health effects. They identified 884 of them as toxic substances. In a 1991 study, the US Environmental Protection Agency found that 100% of the perfumes they tested contained toluene, which can cause liver, kidney and brain damage as well as damage to a developing fetus. Certain fragrance ingredients, such as phthalates, have been found to have hormone disrupting properties. Diethyl phthalate (DEP), a solvent used in fragrances, has been linked to adverse reproductive effects, including DNA damage to human sperm. Many of the fragrance ingredients that have harmful effects are not listed on ingredient labels. In the European Union, labels are required to identify 24 well-known allergenic substances that are used to create fragrances. For more information on hazardous fragrance ingredients, see also phthalates and musks.
Hydroquinone – Hydroquinone is found in many skin lightening products, and is considered to be one of the most toxic ingredients allowed in cosmetics. It can also be present as an impurity in ingredients such as tocopherol acetate, used in facial and skin cleansers and hair conditioners. Hydroquinone works by reducing melanin in the skin, and therefore increases exposure to UVA and UVB rays of the sun. Hydroquinone is classified as a cancer causing agent by Health Canada. It has been linked to kidney damage, and can cause a skin condition called ochronosis in which the skin becomes dark and thick. It was assessed under Canada’s Chemicals Management Plan as a chemical of high concern. As a result, it has been added to Health Canada’s Cosmetic Hotlist and is restricted to use in hair dye and nail products. Warnings on hair dye products should tell users not to dye eyelashes or eyebrows, and users of nail products should be warned to avoid skin contact.
Iodoproponyl Butylcarbamate – Iodoproponyl Butylcarbamate is a preservative found in baby wipes, moisturizers, sunscreens and shampoos, as well as other cosmetic products. It is a pesticide that is registered for use as a fungicide and as a wood preservative. It can cause skin allergies, and may have toxic properties that have not been assessed. It is very toxic when inhaled and should be avoided in aerosol products.
Lead – Lead is a known carcinogen and neurotoxin that can lead to learning and behaviour problems. It has also been linked to reduced fertility. It is readily absorbed through the skin, and accumulates in the bones. Large accumulations can result in leg cramps, muscle weakness, numbness and depression. Lead can be a contaminant in many different kinds of products including sunscreens, foundation, nail colours, whitening toothpaste, and lipstick. A 2008 study by Health Canada found lead in 21 of 26 lipsticks tested. Although lead is prohibited from use in lipsticks, it can be found in colour additives or as impurities in ingredients. It is an ingredient in Grecian Formula 16 and other dark hair dyes for men available in the US, but it is banned from the formulas used in Canada and Europe.
Methylisothiazolinone and Methylchloroisothiazolinone – Methylisothiazolinone and methylchloroisothiazolinone are preservatives used in many cosmetics and personal care products. They are most often found in hair products — shampoos, conditioners and dyes — but they are also used in body washes and cleansers. They have been shown in animal studies to be toxic to the immune system and possibly to the developing nervous system. Health Canada’s Hotlist allows them to be used together in very limited concentrations for rinse-off products and in even smaller concentrations for leave-on products.
Musks – Galaxolide, Tonalide, musk xylene and musk ketone are musks, which are popular replacements for natural ingredients once used as fragrances in cosmetics. Different musks have different hazardous properties. Although data on their toxicity are scarce, some musks appear to have effects on reproduction. A 2009 study of Austrian students detected 11 different musks in their blood. Galaxolide was found in 83% of the students. The highest levels were found in students who used the most lotion and perfume. Canada has restricted the use of two little used musks — musk ambrette and musk tibetene — in cosmetics, but has no restrictions on the more commonly used musks. The European Union has identified musk xylene as a substance of very high concern.
Nanoparticles – Nanoparticles are particles from known chemicals that are manipulated to extremely small dimensions in order to attain certain properties. Widely used in personal care products, particularly sunscreens, the original chemical will be listed on ingredient lists of Canadian cosmetics but there is no requirement to indicate whether it is present in nano form. Nanoparticles are untested for their effects on human health. Their small size means that they can enter the body more easily and have greater access to vulnerable organs and tissues. Animal studies suggest that some nanomaterials in the body cause inflammation, damage brain cells and cause pre-cancerous lesions. The European Union has ruled that companies must indicate when a chemical is used in nano form by adding “nano” in brackets after the chemical’s name on the ingredient list.
Nonylphenol – This estrogen-mimicking chemical is a surfactant used for its detergent properties. It can be found in some plastics, as well as shaving creams, shampoos and hair colours. It can be created when certain chemicals commonly found in personal care products break down. Nonylphenols can be a component in polyvinyl chloride (PVC), a compound often found in acrylic nails. They are persistent in the environment and of such concern that many European countries are phasing them out. Some manufacturers have voluntarily discontinued their use.
Oxybenzone – Oxybenzone, also known as benzophenone-3, 4-MBC and homosalate, is a sunscreen agent and UV light absorber. It is the active ingredient in most sunscreens. The higher the SPF of the sunscreen, the higher the concentration of oxybenzone is likely to be. It is also common in sunscreen moisturizers, facial moisturizers, sunscreen lip balms, skin care lotions, lipstick and hairspray. It is associated with photoallergic reactions in the sun, and is very easily absorbed through the skin. Oxybenzone also assists other ingredients to penetrate the skin. There is scientific evidence suggesting that oxybenzone is a hormone disruptor and may be toxic to the nervous system. A 2008 study from the Mount Sinai School of Medicine found that oxybenzone exposure to pregnant women was associated with low birth weight baby girls. According to the Centers for Disease Control, it is detectable in 97% of people tested in the US.
Parabens – Parabens are preservatives with antibacterial properties. They are widely used in all kinds of personal care products, and particularly deodorants. Paraben on the ingredient list is usually preceded by the prefixes methyl-, ethyl-, butyl-, or propyl-. Parabens mimic estrogen, a hormone that is associated with breast cancer. Parabens can cause allergic reactions or contact dermatitis in some people. Parabens are absorbed through the skin and have been found in biopsied tissue from breast cancer tumours. Safer alternatives to parabens exist, and many products are now marketed as “paraben-free”.
Phenylenediamine – Used in permanent hair dyes, phenylenediamine or PPD is a coal tar dye of particular concern. It is found in most hair dyes, even products marketed as “natural” or “herbal”, and is present in high concentrations in the darkest formulations. It may even be found in hair dyes advertized as “black henna”. Some tattoo artists use it to darken henna tattoos. PPD is a carcinogen, and it can also cause severe skin irritation and react with other chemicals to cause photosensitivity. Health Canada requires warnings about skin irritation and blindness if PPD is present in products used for dying eyelashes or eyebrows.
Phthalates – Everyone in the general population is exposed to phthalates from one source or another. They are found in many products from soft plastics and air fresheners to shampoos and nail polish. Tests done by the Campaign for Safe Cosmetics found phthalates in 3/4 of the 72 products they tested, including deodorants, fragrances, hair gels, mousses, hairsprays and hand and body lotions. Phthalates are used to enhance fragrances to make them last longer and to denature alcohol. Research has shown that phthalates disrupt the hormonal system and interfere with reproduction. A 2008 study published in Environmental Health Perspectives showed that pregnant women exposed to high phthalate levels were more likely to give birth to baby boys with a shortened distance between the anus and the genitals. This study showed that the effects of phthalates on humans were similar to the effects seen in animal studies. The shortened ano-genital distance is associated with genital problems and feminization. An earlier 2002 study in the same journal found that one common type of phthalate, diethyl phthalate (DEP) is damaging to the DNA of sperm in adult men at current levels of exposure. DNA damage to sperm can lead to infertility. DEP is a popular fragrance ingredient and the phthalate that is found in the highest levels in humans. Although some manufacturers have reduced their use of phthalates over the last 8 years, recent product tests found that many fragrances still contain high levels of DEP. Another phthalate, dibutyl phthalate (DBP), has been a favourite ingredient in nail polishes, and US women of child-bearing age have been found to have high levels of DBP. As well, butyl benzyl phthalate (BBP), used to make nail polishes and lipsticks glossy, has been linked in animal studies to an increased risk of breast cancer. For several years, the European Union has banned DBP, BBP and DEHP (di(2-ethylhexyl)phthalate) from use in cosmetics, but in Canada and the US there are no restrictions on any phthalates in cosmetics. In addition, phthalates are difficult to avoid. Except for nail polish, phthalates are not generally listed as ingredients on labels because Canada’s Cosmetic Regulations allow them to be included under the heading of “fragrance”.
Polyethylene Glycol – Polyethylene glycol (PEG) and Ceteareth, both petrochemical compounds, are found in many personal care products, such as body washes, liquid soap, baby wipes, sunscreens and shampoo. They are used as thickeners, softeners, moisture-carriers and penetration enhancers. Both PEG and ceteareth may be contaminated with 1,4-dioxane, a probable carcinogen, and ethylene dioxide, a known human carcinogen. 1,4-dioxane readily penetrates the skin, and is considered unsafe for injured or damaged skin. While 1,4-dioxane can be removed from products easily and economically by vacuum stripping during the manufacturing process, there is no way to determine which products have undergone this process. Labels are not required to show this information.
Polysorbate 60 and Polysorbate 80 – Polysorbate 60 and polysorbate 80 are used as emulsifying agents and fragrance ingredients in many different types of personal care products. These chemicals may be contaminated with 1,4-dioxane, a probable carcinogen, which readily penetrates the skin. While 1,4-dioxane can be removed from products easily and economically by vacuum stripping during the manufacturing process, there is no way to determine which products have undergone this process. Labels are not required to show this information. Ethylene oxide, a known carcinogen, may also be a contaminant of these ingredients.
Propylene Glycol – Propylene glycol is used in many skin products, including moisturizers, facial cleansers, foundations, and anti-aging products, as well as mascara and hair colour products. It is widely used as a moisture-carrying ingredient in place of glycerine because it is cheaper and more readily absorbed through the skin. It is related to polyethylene glycol (PEG) and is known to cause contact dermatitis even at very low concentrations. It is recognized as a neurotoxin by the US National Institute for Occupational Health and Safety, and it may cause kidney damage. The Material Safety Data Sheet for propylene glycol warns workers handling this chemical to avoid skin contact.
Quaternary Ammonium Compounds (Quats) – Listed on labels as benzalkonium chloride, cetrimonium bromide and quaternium-15, these compounds are caustic and can irritate the eyes. Quaternium-15 is a formaldehyde releaser and the number one cause of preservative-related contact dermatitis. For about 5% of people, quats are an extreme sensitizer and can cause a variety of asthma-like symptoms, including respiratory arrest. When they are used with hot running water, steam increases the inhalation of vapours. These compounds are used in a wide range of products as preservatives, surfactants and germicides. They make hair and skin feel softer immediately after use but long-term use will cause dryness.
Selenium Sulfide– Selenium sulfide is an anti-dandruff and hair conditioning agent found in shampoos, conditioners and dandruff treatments. It is believed to be a neurotoxin and it is classified as a possible human carcinogen by both Environment Canada and the US National Toxicology Program.
Sodium Lauryl Sulfate, Sodium Laureth Sulfate – Sodium lauryl sulfate is a known skin and eye irritant and enhances allergic response to other toxins and allergens. It is used as a lathering agent and detergent, and is present in hundreds of commercial shampoos, body washes, and bubble baths, as well as skin creams and some brands of toothpaste. When sodium laurel sulfate is combined with ethylene oxide (ethoxylized) to create the milder sodium laureth sulfate, it may become contaminated with 1,4-dioxane, a probable carcinogen. 1,4-dioxane readily penetrates the skin. While 1,4-dioxane can be removed from products easily and economically by vacuum stripping during the manufacturing process, there is no way to determine which products have undergone this process. Labels are not required to show this information. This processing may also lead to the creation of a known carcinogen, ethylene oxide. Ammonium lauryl sulfate is the same compound as sodium lauryl sulfate and has the same uses, but the sodium group has been replaced with an ammonium atom. Although it is also a skin irritant, ammonium lauryl sulfate is not considered quite as hazardous as sodium lauryl sulfate, unless it is ethoxylized. During its ethoxylation to become ammonium laureth sulfate, it is also likely to be contaminated with 1,4-dioxane, a probable carcinogen, and ethylene oxide, a known carcinogen.
Talc – Talc is a naturally occurring mineral which is carcinogenic when inhaled. In addition, women who regularly use talc in the genital area are at increased risk for ovarian cancer. Airborne talc in body powders and antiperspirant sprays can irritate the lungs. Talcum powder is reported to cause coughing, vomiting, and even pneumonia. Many pediatricians now tell parents to avoid using talc on babies as it can cause respiratory distress. Talc is found in blushes, face powders, eye shadows, foundation and skin fresheners. Used near the eyes, it can irritate sensitive mucous membranes. Talc in liquid cosmetic formulations poses minimal risk.
TEA, DEA and MEA – Triethanolamine (TEA), diethanolamine (DEA) and monoethanolamine (MEA) are additives used in different types of cosmetics such as sunscreens, moisturizers, foundations and hair colour. Other DEA compounds, cocamide DEA and lauramide DEA, are used in soaps, cleansers and shampoos. DEA and its related compounds are used to adjust the pH of products and to act as surfactants. Surfactants help to mix oil and water and work as emulsifiers or wetting agents. DEA and TEA are known to combine with nitrates to form nitrosamines, classified as possible carcinogens by the International Agency for Research on Cancer. If a product contains nitrites (used as preservatives or present as contaminants although not listed on labels), a chemical reaction can occur either during manufacturing or after a product is made. There is no way to know which products contain nitrosamines because the government does not require companies to disclose this information on labels. Repeated skin application of DEA was found to cause liver and kidney damage in animals. Researchers also discovered that when absorbed through the skin, DEA accumulated in organs. TEA, which is also used as a fragrance ingredient, is toxic to the skin, and to the respiratory and immune systems.
Thimerosol– Thimerosol is a mercury-containing preservative. According to Health Canada, mercury may cause allergic reactions, skin irritation and adverse effects on the nervous system. Studies have also show that it can affect reproduction. Thimerosol is used as a preservative for products applied in the area of the eye, such as eye drops and contact lens solutions. Mercury itself is also sometimes found in cosmetics, and in 2010 tests by the Chicago Tribune found a number of skin whitening products contained high levels of mercury. Mercury is readily absorbed through the skin. Thimerosol is on Health Canada’s Cosmetic Hotlist as a prohibited substance.
Titanium Dioxide– Titanium dioxide is widely used in personal care products such as toothpastes to provide whiteness and opacity. It is also used in cosmetic and skin care products and in many sunscreens to protect the skin from ultaviolet light. Titanium dioxide is lcassified as a “possible carcinogen” by the International Agency for Research on Cancer (IARC), but is not considered to be a significant health hazard unless it is in powdered form. However, in recent years almost every sunscreen manufacturer has chosen to use nano-sized particles of titanium dioxide or zinc oxide so that when sunscreen is applied, it appears clear instead of white. The health effects of nano-sized titanium dioxide or other nanoparticles are unknown because of the lack of testing, but the few animal studies that have been done suggest serious concerns that these smaller particles may have greater risks.
Toluene– Toluene is a solvent used in nail polish and nail treatments to suspend colour and form a smooth finish on a nail. It is also listed on labels as methylbenzene or toluol. Exposure to toleuene can irritate the eyes, throat and lungs, and cause dizziness, headaches, fatigue and nausea. It is also known as a reproductive toxin and may put pregnant women at risk of having a baby with birth defects or delayed development. Benzene, which is a known human carcinogen, may be a contaminant of toluene, and may give toluene carcinogenic potential.
Triclosan and Triclocarban– Triclosan and triclocarban are synthetic antibacterial chemicals added to soaps, toothpastes, mouthwash, deodorant, shaving cream and other personal care products. Since they are “antibacterial” and not antiviral, they have no effect on viruses, and are, therefore, not effective against colds and flu. Triclosan, which is more commonly used, has been detected in human breast milk, and in 75% of human tissue samples taken, demonstrating widespread exposure. Studies show that triclosan and triclocarban may have endocrine disrupting effects, and in animal studies triclosan was shown to reduce thyroid hormones, which are critical for normal development. An Advisory Panel to the US Food and Drug Administration has said that there is no evidence that soaps with triclosan are any more effective in killing bacteria than plain soap and water. In 2009, the Canadian Medical Association called on the federal government to ban triclosan in consumer products because it causes bacterial resistance, which can interfere with the effectiveness of antibiotics.