Extracting values that are greater than the threshold

the Alert vector below is a combination of time in hours and minutes(column 1 and 2 respectively) and corresponding oxygen values (column 3). The threshold of oxygen is 21 and i want to store only values greater than 21 and their corresponding times in a (nx3) OxygenAlt vector. When i run the code below, i get the values of the unchanged Alert vector and they are not stored the way i want, please help.

Alert = 1.0000 12.0000 24.3687

2.0000 24.0000 19.3810

3.0000 36.0000 19.5573

4.0000 48.0000 18.9562

6.0000 0 22.3963

7.0000 12.0000 21.6010

8.0000 24.0000 22.6534

9.0000 36.0000 22.0820

10.0000 48.0000 21.1834

12.0000 0 19.5970

13.0000 12.0000 19.5775

14.0000 24.0000 23.5089

15.0000 36.0000 20.4282

16.0000 48.0000 20.8580

18.0000 0 23.5719

19.0000 12.0000 19.1469

20.0000 24.0000 20.9754

21.0000 36.0000 22.6969

when i run this code i get incorrect answers, please help

Alert = [OxygenTime Oxygen]
O = [Oxygen];
for Col = 1:1:length(Oxygen)
    n = Alert(:,3)./21 ;
 if n > 1
        OxygenAlt = [Alert]
 end
end

Answer 1

[Peng Li]

outAlert = Alert(Alert(:, 3) > 21, :);

should be what you want to have

[WILLBES BANDA]

It works perfectly fine, Thank you very very much

Answer 2

[Torsten]

OxygenAlt = Alert(Alert(:,3)>21)

[WILLBES BANDA]

It also works just fine, thank you very much