function validate_option1()
{
    var alertText = "To calculate a wedding gift offset you must:\r\n";
    var option1IsValid = true;
    if (document.getElementById)
    {
        var numericExpression = /^[0-9]+$/;
        
        // validate tonnes of carbon
        if (!document.getElementById('txtOpt1CarbonTns').value.match(numericExpression)) {
            option1IsValid = false;
            alertText += " - Enter carbon emissions tonnes in numeric format\r\n";
        }
        
        // return the option1IsValid, alert user if option1IsValid == false with alertText
        if (option1IsValid) {
            return true;
        } else {
            alert(alertText);
            return false;
        }
    }
}