Abstract

Highlights

• UF_SG did not significantly predict ADHD diagnosis or ADHD-type symptoms.
• Higher tap water fluoride was associated with higher odds of an ADHD diagnosis.
• Higher water fluoride was associated with more ADHD-type symptoms for adolescents.
• Adolescents living in a fluoridated region had higher odds of an ADHD diagnosis.
• Adolescents living in fluoridated regions had more ADHD-type symptoms.

Background

Exposure to fluoride has been linked with increased prevalence of attention deficit hyperactivity disorder (ADHD) in the United States and symptoms of inattention in Mexican children. We examined the association between fluoride exposure and attention outcomes among youth living in Canada.

Method

We used cross-sectional data collected from youth 6 to 17 years of age from the Canadian Health Measures Survey (Cycles 2 and 3). Urinary fluoride concentration adjusted for specific gravity (UFSG) was available for 1877 participants. Water fluoride concentration measured in tap water samples was available for 980 participants. Community water fluoridation (CWF) status was determined by viewing reports on each city’s website or contacting the water treatment plant. We used logistic regression to test the association between the three measures of fluoride exposure and ADHD diagnosis. Linear regression was used to examine the relationship between the three measures of fluoride exposure and the hyperactivity/inattention score on the Strengths and Difficulties Questionnaire (SDQ).

Results

UF_SG did not significantly predict ADHD diagnosis or hyperactive/inattentive symptoms. A 1 mg/L increase in tap water fluoride level was associated with a 6.1 times higher odds of an ADHD diagnosis (95% CI = 1.60, 22.8). A significant interaction between age and tap water fluoride level (p = .03) indicated a stronger association between tap water fluoride and hyperactivity/inattention symptoms among older youth. A 1 mg/L increase in water fluoride level was associated with a 1.5 SDQ score increase (95% CI: 0.23, 2.68, p =.02) for youth at the 75th percentile of age (14 years old). Similarly, there was a significant interaction between age and CWF. At the 75th percentile of age (14 years old), those living in a fluoridated region had a 0.7-point higher SDQ score (95% CI = 0.34, 1.06, p < .01) and the predicted odds of an ADHD diagnosis was 2.8 times greater compared with youth in a non-fluoridated region (aOR = 2.84, 95% CI: 1.40, 5.76, p < .01).

Discussion

Exposure to higher levels of fluoride in tap water is associated with an increased risk of ADHD symptoms and diagnosis of ADHD among Canadian youth, particularly among adolescents. Prospective studies are needed to confirm these results.

Correction to Table 4

Table 4. Adjusted Odds Ratio (aOR) and Beta Coefficient (B) for the association between fluoride exposure and ADHD diagnosis and inattentive/hyperactive symptoms.

	ADHD diagnosis		SDQ H/I subscale score
	aORa	95% CI	Ba	95% CI
UFSG (mg/L) b	0.96	0.63, 1.46	0.31	?0.04, 0.66
Fluoride in tap water (mg/L)	6.10*	1.60, 22.8	0.31*c	0.04, 0.58
75th percentile age	—d	–	1.52*	0.23, 2.80
25th percentile age	–	–	-0.33	-1.51, 0.84
CWF statuse	1.21*	1.03, 1.42	0.11*	0.02, 0.20
75th percentile age	2.84*	1.40, 5.76	0.70*	0.34, 1.06
25th percentile age	0.91	0.41, 1.99	0.04	-0.38, 0.46

Abbreviations: ADHD = Attention Deficit/Hyperactivity Disorder; CWF = community water fluoridation; aOR = adjusted odds ratio; B = beta estimate; SDQ H/I = Strengths and Difficulties Questionnaire Hyperactivity-Inattention.

a

Adjusted for child’s sex, age at interview, ethnicity (white or other), Body Mass Index, highest level of parental education, total household income, exposure to cigarette smoke inside the home (yes/no), concurrent blood lead level (log₁₀-transformed).

b

Non-significant main effect of urinary fluoride level predicting ADHD diagnosis (B = -0.04, p = .84, N = 119/1635 with ADHD/total sample) or SDQ subscale score (B = 0.31, p = .08, N = 1532).

c

Interaction between age and water fluoride level predicting SDQ subscale score (B = 0.31, p = .03, N = 588).

d

Since the interaction between age and water fluoride was not significant, only the main effects are presented for the logistic regression predicting ADHD diagnosis from fluoride in tap water (mg/L; N 39/626 with ADHD/total sample).

e

Significant interaction between age and CWF status predicting ADHD diagnosis (B = 0.19, p = .03, N = 65/1173 with ADHD/total sample) and SDQ subscale score (B = 0.11, p = .01, N = 1118).

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*Corrigendum online at https://www.sciencedirect.com/science/article/pii/S0160412022000174?via%3Dihub

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