Activity in the construction industry, whether measured by new home starts, floor areas of office space built in a period, or employment of construction workers, exhibits both seasonal cycles and sporadic industry-wide recessions. The recessions are usually part of general economic down-turns. They may be due to excessively high mortgage interest rates or unavailability of lending money or both. Construction is affected by prices and by shortages of supplies, notably steel and wood. The instability of the industry exposes millions of construction workers to part-time employment and frequent lay-offs.