def standardize_sample_duration(sample_duration):
    """
    sample_duration: a string description of duration at which devices sample and measure pollutants
    returns a standardized string - '24-hour' or '8-hour' or '1-hour'
    """
    durations = ['24','8','1']
    for duration in durations:
        if duration in sample_duration:
            return duration+'-hour'
    return 'unknown'