Hair Biomarkers Study

Inclusion Criteria: 1. Normal, healthy children aged 6 months to 24 months and their mothers (or father, if mother is not able to participate). 2. Living in Santa Clara County, San Mateo County, or Alameda County in Northern California. 3. Willing to give their home address (in order to receive the study kits at home). 4. Committed to regular follow-up at 6-month intervals for the full study duration of 2 years. Exclusion Criteria: 1. Children with tinea capitis, alopecia areata, scalp eczema, dermatitis, or other scalp conditions. 2. Children exposed to systemic steroid therapy for any diagnosis in the 3 months prior to study entry. 3. Children receiving any other prescription drugs that alter HPA axis function or cortisol release, if the period of exposure is 2 weeks or longer within the 3 months prior to study entry. 4. Children with chronic medical conditions such as cystic fibrosis, sickle cell disease, asthma, eczema, or other chronic conditions. 5. Children with known developmental delay, Trisomy-21 and other chromosomal anomalies, seizure disorders, chronic pain, cerebral palsy, or other disabilities. 6. Children whose hair has received chemical exposures (e.g., dying, bleaching, chemical straightening, perming) in the 90 days prior to study entry or for 90 days before each follow-up appointment during the 2-year study period.